code: def _init_multi_count_metrics(self, pplan_helper):
      
      to_init = [self.metrics[i] for i in self.to_multi_init
                 if i in self.metrics and isinstance(self.metrics[i], MultiCountMetric)]
      for out_stream in pplan_helper.get_my_spout().outputs:
        stream_id = out_stream.stream.id
        for metric in to_init:
          metric.add_key(stream_id)
docs: Initializes the default values for a necessary set of MultiCountMetrics