code: def AddEventTag(self, event_tag):
      
      self._RaiseIfNotWritable()
  
      self._storage_file.AddEventTag(event_tag)
  
      self._session.event_labels_counter['total'] += 1
      for label in event_tag.labels:
        self._session.event_labels_counter[label] += 1
      self.number_of_event_tags += 1
docs: Adds an event tag.
  
      Args:
        event_tag (EventTag): an event tag.
  
      Raises:
        IOError: when the storage writer is closed.
        OSError: when the storage writer is closed.