[Elasticsearch] Log queued requests

[afharo]

Summary

Resolves #152452.

It logs a warning when the HTTP Agent handling Elasticsearch connections has queued requests waiting for an available socket.

This will increase the observability of how the shared HTTP Agents are performing and will unblock the work in #151911.

Checklist

• Unit or functional tests were updated or added to match the most common scenarios

For maintainers

• This was checked for breaking API changes and was labeled appropriately

How to test

Setting elasticsearch.maxSockets: 1 in kibana.yml should make the log show whenever there's anything requesting information from ES (a user is logging in or a dashboard is refreshing).

[afharo]

I moved the calculation logic closer to the Agent Manager so that we didn't need to expose the logger publicly.

At the same time, it feels safer if we are not exposing the agents publicly for anyone else to tamper with.

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 2b2adde

Metrics [docs]

Public APIs missing comments

Total count of every public API that lacks a comment. Target amount is 0. Run node scripts/build_api_docs --plugin [yourplugin] --stats comments for more detailed information.

id	before	after	diff
@kbn/core-elasticsearch-client-server-internal	14	12	-2
@kbn/core-elasticsearch-client-server-mocks	34	33	-1
total			-3

Unknown metric groups

API count

id	before	after	diff
@kbn/core-elasticsearch-client-server-mocks	38	37	-1

ESLint disabled line counts

id	before	after	diff
securitySolution	428	430	+2

Total ESLint disabled count

id	before	after	diff
securitySolution	506	508	+2

History

• 💔 Build #111719 failed 2ab51fb
• 💔 Build #111707 failed 46ef750b4bf874f65d18fe1c830fd481f8e922a2

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @afharo

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)