Added key field to filter bucket agg

[l-trotta]

As reported in a Java client issue, the FiltersBucket aggregation result can have a key field if keyed was set as false in the request.

No server code, key is a common field in aggregation and I couldn't find where it's added to the FiltersAggregation result.

server code

Here's a reproducer:

PUT /logs/_bulk?refresh
{ "index" : { "_id" : 1 } }
{ "body" : "warning: page could not be rendered" }
{ "index" : { "_id" : 2 } }
{ "body" : "authentication error" }
{ "index" : { "_id" : 3 } }
{ "body" : "warning: connection timed out" }

GET logs/_search
{
  "size": 0,
  "aggs" : {
    "messages" : {
      "filters" : {
        "filters" : {
          "errors" :   { "match" : { "body" : "error"   }},
          "warnings" : { "match" : { "body" : "warning" }}
        },
        "keyed": false
      }
    }
  }
}

Response without "keyed": false is:

  "aggregations": {
    "messages": {
      "buckets": {
        "errors": {
          "doc_count": 1
        },
        "warnings": {
          "doc_count": 2
        }
      }
    }
  }

with "keyed": false it's:

  "aggregations": {
    "messages": {
      "buckets": [
        {
          "key": "errors",
          "doc_count": 1
        },
        {
          "key": "warnings",
          "doc_count": 2
        }
      ]
    }
  }

[github-actions]

Following you can find the validation changes against the target branch for the APIs.

No changes detected.

You can validate these APIs yourself by using the make validate target.