[8.x](backport #40360) [kibana] support api_key

[mergify[bot]]

Summary

Adds support for api_key authentication in the kibana module

Checklist

• [x] My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] 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

• create an api key and grab the beats format[1]
• configure kibana module to use that api_key:

  metricbeat.modules:
  - module: kibana
  xpack.enabled: true
  hosts: ["http://localhost:9200"]
  api_key: <key in beats format>
  period: 10s

• kibana metricsets are successfully collecting documents

[1]

Related issues

• Step one to adding api_key to Kibana module: https://github.com/elastic/enhancements/issues/22286

Use cases

Screenshots

Logs

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This is an automatic backport of pull request #40360 done by Mergify.

[mergify[bot]]

This pull request has not been merged yet. Could you please review and merge it @sachin-frayne? 🙏

[mergify[bot]]

This pull request has not been merged yet. Could you please review and merge it @sachin-frayne? 🙏