code: def _add_metric(self, metrics, metric_path, data, data_path):
          
          current_item = data
          for path_element in data_path:
              current_item = current_item.get(path_element)
              if current_item is None:
                  return
  
          self._set_or_sum_metric(metrics, metric_path, current_item)
docs: If the path specified by data_path (a list) exists in data,
          add to metrics.  Use when the data path may not be present