Add ELU fields to metricbeat Kibana module.

[lukeelmers]

This PR updates the Metricbeat kibana module with support for the new event_loop_utilization field added to Kibana in elastic/kibana#153717. We plan to use this metric for autoscaling in serverless, so metricbeat needs to know about it.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

• run kibana from source & ES from snapshot as explained in https://www.elastic.co/guide/en/kibana/master/development-getting-started.html
• pull down this PR, cd metricbeat
• mage build
• ./metricbeat modules enable kibana
• uncomment the elasticsearch.output username/password lines in metricbeat.yml (elastic:changeme)
• update ./modules.d/kibana.yml:

- module: kibana
  metricsets:
    - stats # be sure to enable stats metricset
  period: 10s
  hosts: ["localhost:5601"]
  basepath: "abc" # whatever your base path is
  username: "elastic"
  password: "changeme"

• ./metricbeat -e
• create metricbeat-* data view in kibana
• you should now be seeing some log entries which contain event_loop_utilization

[elasticmachine]

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• Start Time: 2023-04-18T23:59:08.562+0000

• Duration: 51 min 24 sec

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Test	Results
Failed	0
Passed	4367
Skipped	885
Total	5252

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🤖 GitHub comments

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[lukeelmers]

cc @elastic/kibana-core

[klacabane]

LGTM

[klacabane]

The kibana module can either route data to metricbeat-* or .monitoring-kibana-8-mb depending on the xpack.enabled value provided. The monitoring-* template is defined in elasticsearch, could you also add the mappings there ?

[lukeelmers]

Sure thing - opened PR here: elastic/elasticsearch#95388

[lukeelmers]

@elasticmachine run elasticsearch-ci/docs

[lukeelmers]

Thanks @klacabane - looks like I don't have permissions to merge, would you mind merging this when you are ready?

[klacabane]

🚢