code: def last_timestamp(self, event_key=None):
      
      if event_key is None:
        timestamps = [self._trackers[key].first_timestamp
                      for key in self._trackers]
        return max(timestamp for timestamp in timestamps if timestamp >= 0)
      else:
        return self._trackers[event_key].last_timestamp
docs: Obtain the last timestamp.
  
      Args:
        event_key: the type key of the sought events (e.g., constants.NAN_KEY). If
        None, includes all event type keys.
  
      Returns:
        Last (latest) timestamp of all the events of the given type (or all
          event types if event_key is None).