message: Make metrics_utils_test extendable.
diff: @@ -31,16 +31,19 @@ from tensorflow.python.training.server_lib import ClusterSpec
  class MetricUtilsTest(test.TestCase):
  + def get_rpc_layer(self):
  + return 'grpc'
  +
  def testClientMetrics(self):
  metric_utils.enable_metrics = True
  cluster_def = multi_worker_test_base.create_in_process_cluster(
  - num_workers=1, num_ps=1, rpc_layer='grpc')
  + num_workers=1, num_ps=1, rpc_layer=self.get_rpc_layer())
  cluster_def['chief'] = [
  'localhost:%d' % multi_worker_test_base.pick_unused_port()
  ]
  cluster_resolver = SimpleClusterResolver(
  - ClusterSpec(cluster_def), rpc_layer='grpc')
  + ClusterSpec(cluster_def), rpc_layer=self.get_rpc_layer())
  cluster = client.Cluster(cluster_resolver)
  @def_function.function
  