Fix mutex unlocking in monitoring registry

[kvch]

What does this PR do?

This PR adds missing defer keywords so mutexes only after the real critical section.

The problem was reported by golangci linter we use in https://github.com/elastic/elastic-agent-libs.

Why is it important?

Previously in the monitoring registry implementation two mutexes were unlocked right after locking. So the data structures they were supposed to protect were still subject to concurrent access.

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.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[mergify]

This pull request does not have a backport label. Could you fix it @kvch? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

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• Start Time: 2022-03-02T10:30:44.220+0000

• Duration: 139 min 32 sec

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Test	Results
Failed	0
Passed	42995
Skipped	3846
Total	46841

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