Address code review feedback for eval metrics

- Add explicit null guard for details in FlagEvalHook.finallyAfter()
- Add OTEL_EXPORTER_OTLP_ENDPOINT generic env var fallback with
  /v1/metrics path appended (per OTel spec fallback chain)
- Add comments clarifying signal-specific vs generic endpoint behavior

Author: typotter

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

@@ -17,7 +17,7 @@ class FlagEvalHook implements Hook<Object> {
   @Override
   public void finallyAfter(
       HookContext<Object> ctx, FlagEvaluationDetails<Object> details, Map<String, Object> hints) {
-    if (metrics == null) {
+    if (metrics == null || details == null) {
       return;
     }
     try {

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

@@ -21,7 +21,10 @@ class FlagEvalMetrics implements Closeable {
   private static final Duration EXPORT_INTERVAL = Duration.ofSeconds(10);
 
   private static final String DEFAULT_ENDPOINT = "http://localhost:4318/v1/metrics";
+  // Signal-specific env var (used as-is, must include /v1/metrics path)
   private static final String ENDPOINT_ENV = "OTEL_EXPORTER_OTLP_METRICS_ENDPOINT";
+  // Generic env var fallback (base URL, /v1/metrics is appended)
+  private static final String ENDPOINT_GENERIC_ENV = "OTEL_EXPORTER_OTLP_ENDPOINT";
 
   private static final AttributeKey<String> ATTR_FLAG_KEY =
       AttributeKey.stringKey("feature_flag.key");
@@ -40,7 +43,12 @@ class FlagEvalMetrics implements Closeable {
     try {
       String endpoint = System.getenv(ENDPOINT_ENV);
       if (endpoint == null || endpoint.isEmpty()) {
-        endpoint = DEFAULT_ENDPOINT;
+        String base = System.getenv(ENDPOINT_GENERIC_ENV);
+        if (base != null && !base.isEmpty()) {
+          endpoint = base.endsWith("/") ? base + "v1/metrics" : base + "/v1/metrics";
+        } else {
+          endpoint = DEFAULT_ENDPOINT;
+        }
       }
 
       OtlpHttpMetricExporter exporter =