[8.x](backport #41816) move metricbeat command setup out of init

[mergify]

Proposed commit message

Move metricbeat command setup outside of init function. Having the setup inside the init function causes problems when you import metricbeat into other code bases. A specific use case is when you use metricbeat as an otel receiver.

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.

Disruptive User Impact

None.

Author's Checklist

• [ ]

How to test this PR locally

Run unit tests

Related issues

• Relates Add metricbeat receiver and E2E integration test elastic-agent#6138
• Relates add metricbeat receiver #41738

Use cases

Screenshots

Logs

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This is an automatic backport of pull request #41816 done by [Mergify](https://mergify.com).

[botelastic]

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