code: def run_metadata_impl(self, run, tag):
      
      try:
        run_metadata = self._multiplexer.RunMetadata(run, tag)
      except ValueError:
        # TODO(stephanwlee): Should include whether FE is fetching for v1 or v2 RunMetadata
        # so we can remove this try/except.
        tensor_events = self._multiplexer.Tensors(run, tag)
        if tensor_events is None:
          return None
        # Take the first event if there are multiple events written from different
        # steps.
        run_metadata = config_pb2.RunMetadata.FromString(
            tensor_events[0].tensor_proto.string_val[0])
      if run_metadata is None:
        return None
      return (str(run_metadata), 'text/x-protobuf')
docs: Result of the form `(body, mime_type)`, or `None` if no data exists.