Add procs running

[fearful-symmetry]

What does this PR do?

Closes #31074

This adds a little subsection to the system_summary metricset with data on individual threads extracted from /proc/stat

I was a little unsure of where to put this, since it's such a small change that isn't really needed by anything else, so I figured it wasn't worth it to throw it in a separate library somewhere.

Why is it important?

Some users want per-thread metrics, see original issue.

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 (Team:Elastic-Agent)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @fearful-symmetry? 🙏.
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

[elasticmachine]

💚 Build Succeeded

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Build stats

• Start Time: 2022-11-09T16:57:13.442+0000

• Duration: 50 min 47 sec

Test stats 🧪

Test	Results
Failed	0
Passed	3895
Skipped	889
Total	4784

💚 Flaky test report

Tests succeeded.

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[fearful-symmetry]

Github CI lint errors are unfixable; the github runner can't compile with cgo, so it's failing on files that import something with cgo.