Field fetcher performance issue with lots of nested fields

[jimczi]

Elasticsearch Version

main

Installed Plugins

No response

Java Version

bundled

OS Version

Not relevant

Problem Description

Mappings that contain a lot of nested fields at multiple levels suffer from a very slow fields option in the _search request.
The FieldFetcher initialisation seems to create a lot of redundant FieldFetcher for every nested field paths.
The following gist reproduces the issue.
Retrieving a single empty doc without providing any field in the field option takes several seconds due to the initialisation of the nested field fetcher.
I believe that the performance bug was introduced in #67432.
I marked the issue as a performance bug since having lots of nested fields shouldn't make every query using the fields option slow.

Steps to Reproduce

See https://gist.github.com/jimczi/87fd6bb6a4428cccbabb986de917f234

Logs (if relevant)

No response

[elasticsearchmachine]

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

[romseygeek]

I think we can get a quick fix here by only creating the nested fetchers if they are actually required, but we should also make this behave well for requests that specifically ask for nested values. The problem at the moment is that all nested lookups are O(n^2) in the number of nested mappers, which is fine for situations where there are only one of two but not so good for mappings with 50-odd of them.

[romseygeek]

Fixed by #97683