Cherry-pick #22605 to 7.x: Add support for customized monitoring API

[urso]

Cherry-pick of PR #22605 to 7.x branch. Original message:

What does this PR do?

allow customized monitoring api to be added

Why is it important?

allow customized monitoring api to be added

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.

Author's Checklist

N/A

How to test this PR locally

N/A

Use cases

Leverage beats' http server, same port to be used, to expose any other info that user likes to do in a beat implementation or customization.

[elasticmachine]

Pinging @elastic/integrations-services (Team:Services)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: Pull request #23279 updated

• Start Time: 2020-12-24T13:24:04.870+0000

• Duration: 50 min 35 sec

Test stats 🧪

Test	Results
Failed	0
Passed	17462
Skipped	1408
Total	18870

Steps errors

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Terraform Apply on x-pack/metricbeat/module/aws

• Took 0 min 15 sec . View more details on here
• Description: terraform apply -auto-approve

Terraform Apply on x-pack/metricbeat/module/aws

• Took 0 min 15 sec . View more details on here
• Description: terraform apply -auto-approve

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	17462
Skipped	1408
Total	18870

[botelastic]

This pull request doesn't have a Team:<team> label.

[ycombinator]

Backport LGTM.