Add task_manager_utilization metricset to Kibana metricbeat module

[jonathan-buttner]

This PR adds a new metricset to the Metricbeat kibana module. This collects the background worker utilization load from the API added in elastic/kibana#153600. We plan to use this metric for autoscaling in serverless.

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
• pull down this PR, cd metricbeat
• mage build
• ./metricbeat modules enable kibana-xpack
• uncomment the elasticsearch.output username/password lines in metricbeat.yml (elastic:changeme)
• update ./modules.d/kibana-xpack.yml:

- module: kibana
  xpack.enabled: false
  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 task_manager_utilization.load

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @jonathan-buttner? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2023-04-27T12:30:50.874+0000

• Duration: 49 min 24 sec

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Test	Results
Failed	0
Passed	4367
Skipped	885
Total	5252

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[ymao1]

/test

[ymao1]

/test

[ymao1]

@elasticmachine merge upstream

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b collect-task-manager-load upstream/collect-task-manager-load
git merge upstream/main
git push upstream collect-task-manager-load