Add telemetry for retrievers

[pmpailis]

This is to update the information present in ClusterStats for retrievers. The proposed approach is to introduce a new element named retriever under sections that would compute just the top-level usages of a retriever, and additionally a new section named retrievers that would contain the names of all the retrievers used anywhere in the tree, i.e. all top-level usages of a retriever + any nested retrievers used (e.g. children of rrf or text_similarity_reranker)

So assuming that we run a knn retriever search, and an rrf with a standard and a knn as direct children, the suggested output would now be:

{
    "search":
    {
     ...
        "sections":                     
        {
            "retriever": 2             
        },
        "retrievers":                   
        {
            "rrf": 1,
            "knn": 2,
            "standard": 1
        }
    }
}

[github-actions]

Documentation preview:

• ✨ Changed pages

[elasticsearchmachine]

Hi @pmpailis, I've created a changelog YAML for you.

[elasticsearchmachine]

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

[pmpailis]

@elasticmachine update branch

[carlosdelest]

LGTM 👍

Should we add an AbstractBWCSerializationTestCase now that the format has changed for ClusterStats?

[pmpailis]

Should we add an AbstractBWCSerializationTestCase now that the format has changed for ClusterStats?

The main change lies in how SearchUsageStats is structured, which is already covered by a bwc test (SearchUsageStatsTests#testSerializationBWC). I've updated it slightly in 215f120 to better reflect on the used TransportVersions.

@carlosdelest do you think that this would be enough, or should we add a new test class for the top level ClusterStatsResponse ?

[pmpailis]

@elasticmachine update branch

[carlosdelest]

do you think that this would be enough

@pmpailis , absolutely! Thanks for pointing that out 👍

[pmpailis]

@elasticmachine update branch

[pmpailis]

run elasticsearch-ci/part-1

[pmpailis]

@elasticmachine update branch

[pmpailis]

run elasticsearch-ci/packaging-tests-windows-sample

[pmpailis]

@elasticmachine update branch

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	8.x