message: set preference order for overwriting threading options
diff: @@ -3617,8 +3617,10 @@ class Options(options_lib.OptionsBase):
  # else we go with `experimental_threading`.
  if not self.threading._has_default_values():
  pb.threading_options.CopyFrom(self.threading._to_proto()) # pylint: disable=protected-access
  + self.experimental_threading = self.threading
  else:
  pb.threading_options.CopyFrom(self.experimental_threading._to_proto()) # pylint: disable=protected-access
  + self.threading = self.experimental_threading
  return pb
  def _from_proto(self, pb):
  