Fix issue with x-pack/metricbeat not using base settings.

[blakerouse]

Proposed commit message

Fixes issue where x-pack/metricbeat did not use the instance.Settings from the oss metricbeat. Required for the proper initialization functions to be called after the change for supporting agentbeat.

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.

Related issues

• Closes Beats autodiscover error in 8.14.0-SNAPOSHOT cloud-on-k8s#7718

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[elasticmachine]

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• Start Time: 2024-04-16T06:39:22.462+0000

• Duration: 52 min 10 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1886
Skipped	133
Total	2019

💚 Flaky test report

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