opentelemetry: Add Javadoc for target attribute filter

Author: becomeStar

opentelemetry/src/main/java/io/grpc/opentelemetry/GrpcOpenTelemetry.java

@@ -432,7 +432,12 @@ Builder enableTracing(boolean enable) {
     }
 
     /**
-     * Sets an optional filter for the grpc.target attribute.
+     * Sets an optional filter to control recording of the {@code grpc.target} metric attribute.
+     *
+     * <p>If the predicate returns {@code true}, the original target is recorded. Otherwise,
+     * the target is recorded as {@code "other"} to limit metric cardinality.
+     *
+     * <p>If unset, all targets are recorded as-is.
      */
     public Builder targetAttributeFilter(@Nullable Predicate<String> filter) {
       this.targetAttributeFilter = filter;

opentelemetry/src/main/java/io/grpc/opentelemetry/OpenTelemetryMetricsModule.java

@@ -69,6 +69,10 @@
  * tracer. It's the tracer that reports per-attempt stats, and the factory that reports the stats
  * of the overall RPC, such as RETRIES_PER_CALL, to OpenTelemetry.
  *
+ * <p>This module optionally applies a target attribute filter to limit the cardinality of
+ * the {@code grpc.target} attribute in client-side metrics by mapping disallowed targets
+ * to a stable placeholder value.
+ *
  * <p>On the server-side, there is only one ServerStream per each ServerCall, and ServerStream
  * starts earlier than the ServerCall. Therefore, only one tracer is created per stream/call, and
  * it's the tracer that reports the summary to OpenTelemetry.