code: def __get_query_agg_avg(cls, field, agg_id=None):
          
          if not agg_id:
              agg_id = cls.AGGREGATION_ID
          query_agg = A("avg", field=field)
          return (agg_id, query_agg)
docs: Create an es_dsl aggregation object for getting the average value of a field.
  
          :param field: field from which the get the average value
          :return: a tuple with the aggregation id and es_dsl aggregation object. Ex:
                  {
                      "avg": {
                          "field": <field>
                  }