Acknowledge the given messages.

        Args:
            items(Sequence[AckRequest]): The items to acknowledge.

def ack(self, items):
        """Acknowledge the given messages.

        Args:
            items(Sequence[AckRequest]): The items to acknowledge.
        """
        # If we got timing information, add it to the histogram.
        for item in items:
            time_to_ack = item.time_to_ack
            if time_to_ack is not None:
                self._manager.ack_histogram.add(time_to_ack)

        ack_ids = [item.ack_id for item in items]
        request = types.StreamingPullRequest(ack_ids=ack_ids)
        self._manager.send(request)

        # Remove the message from lease management.
        self.drop(items)