Top hits metric aggregation returning malformed responses

[nreese]

Steps to reproduce

1. Install web logs sample data set
2. create new pie chart visualization.
3. split on field machine.os.keyword
4. Configure metric aggregation to be top hits on bytes field.
5. Apply changes
6. Notice that no pie chart is displayed. No errors are given in console

Opening inspector reveals some odd behavior. Notice in the request that top hit field is requested as docvalue_field with use_field_mapping. Then notice that the response is not a number but rather a string like use_field_mapping7616

Request

{
  "aggs": {
    "2": {
      "terms": {
        "field": "machine.os.keyword",
        "order": {
          "_key": "desc"
        },
        "size": 5
      },
      "aggs": {
        "1": {
          "top_hits": {
            "docvalue_fields": [
              {
                "field": "bytes",
                "format": "use_field_mapping"
              }
            ],
            "_source": "bytes",
            "size": 1,
            "sort": [
              {
                "timestamp": {
                  "order": "desc"
                }
              }
            ]
          }
        }
      }
    }
  },

Response

"aggregations": {
    "2": {
      "doc_count_error_upper_bound": 0,
      "sum_other_doc_count": 0,
      "buckets": [
        {
          "1": {
            "hits": {
              "total": 2,
              "max_score": null,
              "hits": [
                {
                  "_index": "kibana_sample_data_logs",
                  "_id": "m_GynnIBZY7Hk4-PIA1x",
                  "_score": null,
                  "_source": {
                    "bytes": 7616
                  },
                  "fields": {
                    "bytes": [
                      "use_field_mapping7616"
                    ]
                  },
                  "sort": [
                    1591887117359
                  ]
                }
              ]
            }
          },
          "key": "win xp",
          "doc_count": 2
        },

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[nreese]

seems related to #68883

[nreese]

cc @lukeelmers

[sulemanof]

Fixed in #71168
Tested on master, works as expected: