code: def keys(self):
          
          keys = []
          for series in self._get_series():
              keys.append(
                  (series.get('measurement',
                              series.get('name', 'results')),
                   series.get('tags', None))
              )
          return keys
docs: Return the list of keys in the ResultSet.
  
          :return: List of keys. Keys are tuples (series_name, tags)