[ML] AIOps: Use field caps option include_empty_fields=false instead of custom query.

[walterra]

Summary

Part of #178606.

As of elastic/elasticsearch#103651 there is a new field caps option include_empty_fields. This PR updates AIOps Log Rate Analysis to make use of this option instead of a custom query and code that identified populated fields.

Checklist

• Unit or functional tests were updated or added to match the most common scenarios
• Flaky Test Runner was used on any tests changed
  ✅ https://buildkite.com/elastic/kibana-flaky-test-suite-runner/builds/5482
• This was checked for breaking API changes and was labeled appropriately

[kibana-ci]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: a23f47e

Failed CI Steps

• FTR Configs #49

Metrics [docs]

✅ unchanged

History

• 💔 Build #197632 failed 8c8c87b

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

cc @walterra

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[qn895]

Code LGTM 🎉

[darnautov]

LGTM