docs: def DeleteOldClientStats(self, yield_after_count,
                             retention_time
                            ):
      """Deletes ClientStats older than a given timestamp."""
      deleted_count = 0
      yielded = False
  
      for stats_dict in itervalues(self.client_stats):
        for timestamp in list(stats_dict.keys()):
          if timestamp < retention_time:
            del stats_dict[timestamp]
            deleted_count += 1
  
            if deleted_count >= yield_after_count:
              yield deleted_count
              yielded = True
              deleted_count = 0
  
      if deleted_count > 0 or not yielded:
        yield deleted_count