Result of the form `(body, mime_type)`, or `None` if no data exists.

def run_metadata_impl(self, run, tag):
    """Result of the form `(body, mime_type)`, or `None` if no data exists."""
    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')