code: def aggregate_stat(origin_stat, new_stat):
      
      for key in new_stat:
          if key not in origin_stat:
              origin_stat[key] = new_stat[key]
          elif key == "start_at":
              # start datetime
              origin_stat[key] = min(origin_stat[key], new_stat[key])
          else:
              origin_stat[key] += new_stat[key]
docs: aggregate new_stat to origin_stat.
  
      Args:
          origin_stat (dict): origin stat dict, will be updated with new_stat dict.
          new_stat (dict): new stat dict.