code: def list_processed_parameter_group_histogram(self, group=None, start=None, stop=None, merge_time=20):
          params = {}
          if group is not None:
              params['group'] = group
          if start is not None:
              params['start'] = to_isostring(start)
          if stop is not None:
              params['stop'] = to_isostring(stop)
          if merge_time is not None:
              params['mergeTime'] = int(merge_time * 1000)
          return pagination.Iterator(
              client=self._client,
              path='/archive/{}/parameter-index'.format(self._instance),
              params=params,
              response_class=archive_pb2.IndexResponse,
              items_key='group',
              item_mapper=IndexGroup,
          )