code: func (a *TermsAggregation) OrderByAggregationAndMetric(aggName, metric string, asc bool) *TermsAggregation {
  	// {
  	//     "aggs" : {
  	//         "genders" : {
  	//             "terms" : {
  	//                 "field" : "gender",
  	//                 "order" : { "height_stats.avg" : "desc" }
  	//             },
  	//             "aggs" : {
  	//                 "height_stats" : { "stats" : { "field" : "height" } }
  	//             }
  	//         }
  	//     }
  	// }
  	a.order = append(a.order, TermsOrder{Field: aggName + "." + metric, Ascending: asc})
  	return a
  }
docs: // OrderByAggregationAndMetric creates a bucket ordering strategy which
  // sorts buckets based on a multi-valued calc get.