[libbeat] Fix and test initialization of queue monitors

[faec]

A stray := instead of = overwrote the intended queue metrics namespace. This PR fixes it and adds a test to make sure the correct namespace is used.

Fixes #40477

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.
If this is a bug or security fix, could you label this PR @faec? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[cmacknz]

Once these are working, do we have to follow up and add them to the elastic_agent package as well so they are visualized in Fleet? https://github.com/elastic/integrations/tree/main/packages/elastic_agent

[faec]

Once these are working, do we have to follow up and add them to the elastic_agent package as well so they are visualized in Fleet?

Yes, #39774 just added raw metrics to libbeat, it didn't add visualizations for any of the new fields.

[cmacknz]

Cool, make sure there is a follow up issue filed so we can make use of the new metrics once they are available, if you aren't planning to just take care of the update.