docs: def get_metric_index(self, metric_cls):
          """
          Get the index name with the data for a metric class
          :param metric_cls: a metric class
          :return: the name of the index with the data for the metric
          """
          ds = self.class2ds[metric_cls.ds]
          if self.index_dict[ds]:
              index_name = self.index_dict[ds]
          else:
              index_name = self.ds2index[metric_cls.ds]
          return index_name