Fix NoClassDefFoundError when OTel SDK absent from classpath

When the OTel SDK jars are not on the application classpath,
loading FlagEvalMetrics fails because field types reference
OTel SDK classes (SdkMeterProvider). This propagated as an
uncaught NoClassDefFoundError from the Provider constructor,
crashing provider initialization.

Fix:
- Change meterProvider field type from SdkMeterProvider to
  Closeable (always on classpath), use local SdkMeterProvider
  variable inside try block
- Catch NoClassDefFoundError in Provider constructor when
  creating FlagEvalMetrics
- Null-safe getProviderHooks() and shutdown() when metrics
  is null

Author: typotter

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

@@ -37,7 +37,9 @@ class FlagEvalMetrics implements Closeable {
       AttributeKey.stringKey("feature_flag.result.allocation_key");
 
   private volatile LongCounter counter;
-  private volatile SdkMeterProvider meterProvider;
+  // Typed as Closeable to avoid loading SdkMeterProvider at class-load time
+  // when the OTel SDK is absent from the classpath
+  private volatile java.io.Closeable meterProvider;
 
   FlagEvalMetrics() {
     try {
@@ -57,9 +59,11 @@ class FlagEvalMetrics implements Closeable {
       PeriodicMetricReader reader =
           PeriodicMetricReader.builder(exporter).setInterval(EXPORT_INTERVAL).build();
 
-      meterProvider = SdkMeterProvider.builder().registerMetricReader(reader).build();
+      SdkMeterProvider sdkMeterProvider =
+          SdkMeterProvider.builder().registerMetricReader(reader).build();
+      meterProvider = sdkMeterProvider;
 
-      Meter meter = meterProvider.meterBuilder(METER_NAME).build();
+      Meter meter = sdkMeterProvider.meterBuilder(METER_NAME).build();
       counter =
           meter
               .counterBuilder(METRIC_NAME)
@@ -113,7 +117,7 @@ public void close() {
 
   void shutdown() {
     counter = null;
-    SdkMeterProvider mp = meterProvider;
+    java.io.Closeable mp = meterProvider;
     if (mp != null) {
       meterProvider = null;
       try {

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

@@ -42,8 +42,14 @@ public Provider(final Options options) {
   Provider(final Options options, final Evaluator evaluator) {
     this.options = options;
     this.evaluator = evaluator;
-    this.flagEvalMetrics = new FlagEvalMetrics();
-    this.flagEvalHook = new FlagEvalHook(flagEvalMetrics);
+    FlagEvalMetrics metrics = null;
+    try {
+      metrics = new FlagEvalMetrics();
+    } catch (NoClassDefFoundError | Exception e) {
+      // OTel classes not on classpath — metrics disabled
+    }
+    this.flagEvalMetrics = metrics;
+    this.flagEvalHook = new FlagEvalHook(metrics);
   }
 
   @Override
@@ -86,12 +92,17 @@ private Evaluator buildEvaluator() throws Exception {
 
   @Override
   public List<Hook> getProviderHooks() {
+    if (flagEvalHook == null) {
+      return Collections.emptyList();
+    }
     return Collections.singletonList(flagEvalHook);
   }
 
   @Override
   public void shutdown() {
-    flagEvalMetrics.shutdown();
+    if (flagEvalMetrics != null) {
+      flagEvalMetrics.shutdown();
+    }
     if (evaluator != null) {
       evaluator.shutdown();
     }