docs: Add javadoc

Author: blakeli0

sdk-platform-java/gax-java/gax/src/main/java/com/google/api/gax/tracing/ApiTracerFactory.java

@@ -73,6 +73,12 @@ default ApiTracer newTracer(ApiTracer parent, ApiTracerContext tracerContext) {
     return newTracer(parent, spanName, tracerContext.operationType());
   }
 
+  /**
+   * Indicates whether this factory requires an {@link ApiTracerContext} to be injected via {@link
+   * #withContext(ApiTracerContext)} before creating tracers.
+   *
+   * @return {@code true} if an {@link ApiTracerContext} should be injected, {@code false} otherwise.
+   */
   default boolean needsContext() {
     return false;
   }