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Add flag evaluation metrics via OTel counter and OpenFeature Hook
Record a `feature_flag.evaluations` OTel counter on every flag evaluation using an OpenFeature `finallyAfter` hook. The hook captures all evaluation paths including type mismatches that occur above the provider level. Attributes: feature_flag.key, feature_flag.result.variant, feature_flag.result.reason, error.type (on error), feature_flag.result.allocation_key (when present). Counter is a no-op when DD_METRICS_OTEL_ENABLED is false or opentelemetry-api is absent from the classpath.
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‎products/feature-flagging/feature-flagging-api/build.gradle.kts‎

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@@ -44,8 +44,10 @@ dependencies {
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api("dev.openfeature:sdk:1.20.1")
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compileOnly(project(":products:feature-flagging:feature-flagging-bootstrap"))
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compileOnly("io.opentelemetry:opentelemetry-api:1.47.0")
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testImplementation(project(":products:feature-flagging:feature-flagging-bootstrap"))
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testImplementation("io.opentelemetry:opentelemetry-api:1.47.0")
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testImplementation(libs.bundles.junit5)
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testImplementation(libs.bundles.mockito)
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testImplementation(libs.moshi)

‎products/feature-flagging/feature-flagging-api/src/main/java/datadog/trace/api/openfeature/FlagEvalHook.java‎

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package datadog.trace.api.openfeature;
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import dev.openfeature.sdk.FlagEvaluationDetails;
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import dev.openfeature.sdk.Hook;
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import dev.openfeature.sdk.HookContext;
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import dev.openfeature.sdk.ImmutableMetadata;
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import java.util.Map;
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class FlagEvalHook implements Hook<Object> {
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private final FlagEvalMetrics metrics;
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FlagEvalHook(FlagEvalMetrics metrics) {
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this.metrics = metrics;
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}
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@Override
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public void finallyAfter(
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HookContext<Object> ctx, FlagEvaluationDetails<Object> details, Map<String, Object> hints) {
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if (metrics == null) {
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return;
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}
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try {
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String flagKey = details.getFlagKey();
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String variant = details.getVariant();
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String reason = details.getReason();
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dev.openfeature.sdk.ErrorCode errorCode = details.getErrorCode();
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String allocationKey = null;
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ImmutableMetadata metadata = details.getFlagMetadata();
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if (metadata != null) {
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allocationKey = metadata.getString("allocationKey");
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}
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metrics.record(flagKey, variant, reason, errorCode, allocationKey);
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} catch (Exception e) {
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// Never let metrics recording break flag evaluation
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}
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}
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}

‎products/feature-flagging/feature-flagging-api/src/main/java/datadog/trace/api/openfeature/FlagEvalMetrics.java‎

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package datadog.trace.api.openfeature;
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import dev.openfeature.sdk.ErrorCode;
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import io.opentelemetry.api.GlobalOpenTelemetry;
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import io.opentelemetry.api.common.AttributeKey;
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import io.opentelemetry.api.common.Attributes;
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import io.opentelemetry.api.common.AttributesBuilder;
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import io.opentelemetry.api.metrics.LongCounter;
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import io.opentelemetry.api.metrics.Meter;
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class FlagEvalMetrics {
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private static final String METER_NAME = "ddtrace.openfeature";
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private static final String METRIC_NAME = "feature_flag.evaluations";
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private static final String METRIC_UNIT = "{evaluation}";
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private static final String METRIC_DESC = "Number of feature flag evaluations";
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private static final AttributeKey<String> ATTR_FLAG_KEY =
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AttributeKey.stringKey("feature_flag.key");
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private static final AttributeKey<String> ATTR_VARIANT =
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AttributeKey.stringKey("feature_flag.result.variant");
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private static final AttributeKey<String> ATTR_REASON =
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AttributeKey.stringKey("feature_flag.result.reason");
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private static final AttributeKey<String> ATTR_ERROR_TYPE = AttributeKey.stringKey("error.type");
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private static final AttributeKey<String> ATTR_ALLOCATION_KEY =
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AttributeKey.stringKey("feature_flag.result.allocation_key");
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private volatile LongCounter counter;
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FlagEvalMetrics() {
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try {
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Meter meter = GlobalOpenTelemetry.getMeterProvider().meterBuilder(METER_NAME).build();
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counter =
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meter
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.counterBuilder(METRIC_NAME)
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.setUnit(METRIC_UNIT)
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.setDescription(METRIC_DESC)
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.build();
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} catch (NoClassDefFoundError | Exception e) {
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// OTel API not on classpath or initialization failed — counter stays null (no-op)
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counter = null;
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}
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}
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/** Package-private constructor for testing with a mock counter. */
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FlagEvalMetrics(LongCounter counter) {
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this.counter = counter;
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}
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void record(
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String flagKey, String variant, String reason, ErrorCode errorCode, String allocationKey) {
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LongCounter c = counter;
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if (c == null) {
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return;
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}
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try {
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AttributesBuilder builder =
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Attributes.builder()
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.put(ATTR_FLAG_KEY, flagKey)
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.put(ATTR_VARIANT, variant != null ? variant : "")
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.put(ATTR_REASON, reason != null ? reason.toLowerCase() : "unknown");
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if (errorCode != null) {
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builder.put(ATTR_ERROR_TYPE, errorCode.name().toLowerCase());
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}
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if (allocationKey != null && !allocationKey.isEmpty()) {
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builder.put(ATTR_ALLOCATION_KEY, allocationKey);
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}
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c.add(1, builder.build());
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} catch (Exception e) {
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// Never let metrics recording break flag evaluation
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}
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}
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void shutdown() {
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counter = null;
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}
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}

‎products/feature-flagging/feature-flagging-api/src/main/java/datadog/trace/api/openfeature/Provider.java‎

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@@ -5,6 +5,7 @@
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import de.thetaphi.forbiddenapis.SuppressForbidden;
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import dev.openfeature.sdk.EvaluationContext;
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import dev.openfeature.sdk.EventProvider;
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import dev.openfeature.sdk.Hook;
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import dev.openfeature.sdk.Metadata;
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import dev.openfeature.sdk.ProviderEvaluation;
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import dev.openfeature.sdk.ProviderEvent;
@@ -14,6 +15,8 @@
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import dev.openfeature.sdk.exceptions.OpenFeatureError;
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import dev.openfeature.sdk.exceptions.ProviderNotReadyError;
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import java.lang.reflect.Constructor;
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import java.util.Collections;
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import java.util.List;
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import java.util.concurrent.TimeUnit;
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import java.util.concurrent.atomic.AtomicBoolean;
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@@ -25,6 +28,8 @@ public class Provider extends EventProvider implements Metadata {
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private volatile Evaluator evaluator;
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private final Options options;
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private final AtomicBoolean initialized = new AtomicBoolean(false);
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private final FlagEvalMetrics flagEvalMetrics;
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private final FlagEvalHook flagEvalHook;
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public Provider() {
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this(DEFAULT_OPTIONS, null);
@@ -37,6 +42,8 @@ public Provider(final Options options) {
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Provider(final Options options, final Evaluator evaluator) {
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this.options = options;
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this.evaluator = evaluator;
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this.flagEvalMetrics = new FlagEvalMetrics();
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this.flagEvalHook = new FlagEvalHook(flagEvalMetrics);
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}
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@Override
@@ -77,8 +84,14 @@ private Evaluator buildEvaluator() throws Exception {
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return (Evaluator) ctor.newInstance((Runnable) this::onConfigurationChange);
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}
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@Override
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public List<Hook> getProviderHooks() {
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return Collections.singletonList(flagEvalHook);
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}
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@Override
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public void shutdown() {
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flagEvalMetrics.shutdown();
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if (evaluator != null) {
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evaluator.shutdown();
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}

‎products/feature-flagging/feature-flagging-api/src/test/java/datadog/trace/api/openfeature/FlagEvalHookTest.java‎

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package datadog.trace.api.openfeature;
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import static org.mockito.ArgumentMatchers.eq;
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import static org.mockito.ArgumentMatchers.isNull;
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import static org.mockito.Mockito.mock;
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import static org.mockito.Mockito.verify;
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import static org.mockito.Mockito.verifyNoInteractions;
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import dev.openfeature.sdk.ErrorCode;
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import dev.openfeature.sdk.FlagEvaluationDetails;
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import dev.openfeature.sdk.ImmutableMetadata;
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import dev.openfeature.sdk.Reason;
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import java.util.Collections;
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import org.junit.jupiter.api.Test;
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class FlagEvalHookTest {
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@Test
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void finallyAfterRecordsBasicEvaluation() {
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FlagEvalMetrics metrics = mock(FlagEvalMetrics.class);
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FlagEvalHook hook = new FlagEvalHook(metrics);
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FlagEvaluationDetails<Object> details =
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FlagEvaluationDetails.<Object>builder()
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.flagKey("my-flag")
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.value("on-value")
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.variant("on")
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.reason(Reason.TARGETING_MATCH.name())
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.flagMetadata(
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ImmutableMetadata.builder().addString("allocationKey", "default-alloc").build())
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.build();
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hook.finallyAfter(null, details, Collections.emptyMap());
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verify(metrics)
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.record(
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eq("my-flag"),
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eq("on"),
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eq(Reason.TARGETING_MATCH.name()),
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isNull(),
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eq("default-alloc"));
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}
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@Test
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void finallyAfterRecordsErrorEvaluation() {
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FlagEvalMetrics metrics = mock(FlagEvalMetrics.class);
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FlagEvalHook hook = new FlagEvalHook(metrics);
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FlagEvaluationDetails<Object> details =
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FlagEvaluationDetails.<Object>builder()
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.flagKey("missing-flag")
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.value("default")
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.reason(Reason.ERROR.name())
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.errorCode(ErrorCode.FLAG_NOT_FOUND)
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.build();
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hook.finallyAfter(null, details, Collections.emptyMap());
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verify(metrics)
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.record(
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eq("missing-flag"),
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isNull(),
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eq(Reason.ERROR.name()),
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eq(ErrorCode.FLAG_NOT_FOUND),
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isNull());
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}
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@Test
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void finallyAfterHandlesNullFlagMetadata() {
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FlagEvalMetrics metrics = mock(FlagEvalMetrics.class);
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FlagEvalHook hook = new FlagEvalHook(metrics);
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FlagEvaluationDetails<Object> details =
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FlagEvaluationDetails.<Object>builder()
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.flagKey("my-flag")
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.value(true)
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.variant("on")
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.reason(Reason.TARGETING_MATCH.name())
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.build();
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hook.finallyAfter(null, details, Collections.emptyMap());
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verify(metrics)
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.record(eq("my-flag"), eq("on"), eq(Reason.TARGETING_MATCH.name()), isNull(), isNull());
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}
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@Test
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void finallyAfterHandlesNullVariantAndReason() {
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FlagEvalMetrics metrics = mock(FlagEvalMetrics.class);
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FlagEvalHook hook = new FlagEvalHook(metrics);
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FlagEvaluationDetails<Object> details =
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FlagEvaluationDetails.<Object>builder().flagKey("my-flag").value("default").build();
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hook.finallyAfter(null, details, Collections.emptyMap());
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verify(metrics).record(eq("my-flag"), isNull(), isNull(), isNull(), isNull());
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}
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@Test
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void finallyAfterNeverThrows() {
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FlagEvalMetrics metrics = mock(FlagEvalMetrics.class);
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FlagEvalHook hook = new FlagEvalHook(metrics);
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// Should not throw even with completely null inputs
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hook.finallyAfter(null, null, null);
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verifyNoInteractions(metrics);
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}
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@Test
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void finallyAfterIsNoOpWhenMetricsIsNull() {
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FlagEvalHook hook = new FlagEvalHook(null);
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FlagEvaluationDetails<Object> details =
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FlagEvaluationDetails.<Object>builder()
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.flagKey("my-flag")
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.value(true)
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.variant("on")
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.reason(Reason.TARGETING_MATCH.name())
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.build();
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// Should not throw
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hook.finallyAfter(null, details, Collections.emptyMap());
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}
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}

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