Fix nested aggregation top_hits with query inner_hits

[mayya-sharipova]

Currently top_hits aggs inside nested aggs inherit the parent
query's inner_hits context, causing an error "Error retrieving path".
This happenes because nested top_hits is already operating on nested child docs,
and asking it to retrieve inner_hits adds an extra incorrect level of nestedness.

This fix proposes to create an empty InnerHitsContext for this case.

Closes #136893

[elasticsearchmachine]

Hi @mayya-sharipova, I've created a changelog YAML for you.

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[benwtrent]

@mayya-sharipova maybe I misunderstand how this will look for users.

But as a user, I would still expect to get inner hits for both whenever I request them. One that is present within _source the other that is present within the aggregator results. Especially since they will likely be different documents (top-hits under a terms agg will definitely return different inner hits than those that are present in the most relevant documents given the top level query).

[mayya-sharipova]

@benwtrent Thanks for your comment. This fix applies only when top_hits agg is a part of nested. When we do nested top_hits agg we already retrieving nested child docs.
So we discussed this offline:

• Documentation how nested agg works with top_hits agg
• We should first check if there is second level of nestedness and allow inner_hits, if not it makes sense to remove them.

[mayya-sharipova]

@benwtrent I've checked an it looks like inner_hits for double nested fields in top_hits doesn't work (for most docs doesn't return anything).
So something like this doesn't work:

PUT comments
{
    "mappings": {
        "properties": {
            "product": {
                "type": "keyword"
            },
            "comments": {
                "type": "nested",
                "properties": {
                  "comment": {
                    "type": "text"
                    },
                    "user": {
                      "type" : "keyword"
                    },
                    "ratings": {
                      "type": "nested",
                        "properties": {
                            "rating": {
                                "type": "integer"
                            },
                            "user": {
                              "type": "keyword"
                            }
                        }
                    }
                }
            }
        }
    }
}

POST /comments/_search?_source=false
{
  "aggs": {
    "nested_ratings": {
      "nested": {
        "path": "comments"  
      },
      "aggs": {
        "ratings_top_hits": {
            "top_hits": {
              "size": 10
            }
        }
      }
    }
  },
  "query": {
      "nested": {
          "inner_hits": {},
          "path": "comments",
          "query": {
              "match_all": {}
          }
      }
  }
}