code: def close(self, end_time=None):
          
          super(Subsegment, self).close(end_time)
          self.parent_segment.decrement_ref_counter()
docs: Close the trace entity by setting `end_time`
          and flip the in progress flag to False. Also decrement
          parent segment's ref counter by 1.
  
          :param int end_time: Epoch in seconds. If not specified
              current time will be used.