Deprecate & remove undocumented alternatives to the nodes hot threads API

[DaveCTurner]

The nodes hot threads API has some little-known synonyms:

elasticsearch/server/src/main/java/org/elasticsearch/rest/action/admin/cluster/RestNodesHotThreadsAction.java

Lines 43 to 54 in a11ccb8

	@Override
	public List<Route> routes() {
	return List.of(
	new Route(GET, "/_cluster/nodes/hotthreads"),
	new Route(GET, "/_cluster/nodes/hot_threads"),
	new Route(GET, "/_cluster/nodes/{nodeId}/hotthreads"),
	new Route(GET, "/_cluster/nodes/{nodeId}/hot_threads"),
	new Route(GET, "/_nodes/hotthreads"),
	new Route(GET, "/_nodes/hot_threads"),
	new Route(GET, "/_nodes/{nodeId}/hotthreads"),
	new Route(GET, "/_nodes/{nodeId}/hot_threads"));
	}

These date back to its introduction in v0.20 but I do not think they have ever been documented. They seem unnecessary and I think we should remove them. There is of course a risk that someone out there is using one of these undocumented endpoints (perhaps due to a typo) and this is a breaking change so we should deprecate them first of course.

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/REST API)

[muachilin]

I would like to work on this! Thanks!

[DaveCTurner]

Thanks @muachilin, go ahead. Note that this'll take two PRs in sequence: the first to deprecate these endpoints and the second to remove them, and both should be opened against the master branch. We will take care of backporting the first. See e.g. #50499 for an example of a PR that deprecates things in the REST API, and note particularly that this includes a test showing that we do indeed log the right warnings when the deprecated API is used.

[atmask]

Hi, @DaveCTurner I was looking to help on this issue but it looks as though the endpoints are already deprecated?
https://github.com/elastic/elasticsearch/blob/master/rest-api-spec/src/main/resources/rest-api-spec/api/nodes.hot_threads.json#L104

[muachilin]

I am already working on this to add tests and deprecate warnings for this file!

[muachilin]

@atmask Do you want to work on this? But I've claimed this issue

[atmask]

@muachilin Ah I see, I didn't realize the issue was actively being taken care of! All yours!

[muachilin]

@atmask Thank you very much!

[DaveCTurner]

Interesting observation @atmask, thanks. I had indeed not noticed that these endpoints were documented and deprecated, in the REST API spec (see #39063). We also need the deprecation warnings in the REST handler too since there's a risk that clients won't all notice them. For instance, as noted in #39063 (comment), Kibana's Console client ignores deprecations in the REST API spec but does respect deprecation warnings emitted by the handlers themselves.

[muachilin]

@DaveCTurner I see! I have added warnings and made some tests about this. The related PR is #52930.

[jasontedor]

This was deprecated in 7.7.0 in #52930. @muachilin are you interested in proceeding with the removal of the deprecated endpoints in the master branch?