Add basic metrics reporting to the memqueue

[fearful-symmetry]

What does this PR do?

First step towards #31113

This adds the basic framework for reporting queue metrics to the memqueue in libbeat. Right now this only adds the two most basic metrics, as I wanted to get feedback from @faec and make sure I was on the right track before I got into anything more in-depth.

This also appends a somewhat simplistic change to an existing test, which should probably be broken into a separate test on its own later.

Why is it important?

This helps address the queue metrics defined in #31113

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]

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• Start Time: 2022-05-31T17:38:50.772+0000

• Duration: 74 min 27 sec

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Test	Results
Failed	0
Passed	22256
Skipped	1935
Total	24191

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[elasticmachine]

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

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b memqueue-metrics upstream/memqueue-metrics
git merge upstream/main
git push upstream memqueue-metrics

[faec]

len(l.buf.entries) returns the total capacity of the queue, not its current number of events. The current event count (including events that have been read by consumers but not yet acked/removed from the queue) would be l.buf.regA.size + l.buf.regB.size (it probably makes sense to make this a helper function on ringBuffer since it's not obvious).

[fearful-symmetry]

That is indeed not obvious, thanks. :D

[cmacknz]

Update or rebase with main to fix the CI build errors.