code: def ack(self, items):
          
          # 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)
docs: Acknowledge the given messages.
  
          Args:
              items(Sequence[AckRequest]): The items to acknowledge.