Trigger all callbacks registered to this Future.

        This method is called internally by the batch once the batch
        completes.

        Args:
            message_id (str): The message ID, as a string.

def _trigger(self):
        """Trigger all callbacks registered to this Future.

        This method is called internally by the batch once the batch
        completes.

        Args:
            message_id (str): The message ID, as a string.
        """
        self._completed.set()
        for callback in self._callbacks:
            callback(self)