Move registry used by handles processor

[fearful-symmetry]

What does this PR do?

Fix for a bug reported by @cmacknz where the migration process of metrics from libbeat to here also changed the location of the handles monitoring metrics. This changes it back from "system" to "beat". We don't have any tests here, so a tad hard to validate this until it makes its way up to the beats repo again.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.md

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2022-06-29T21:57:41.246+0000

• Duration: 46 min 51 sec

Test stats 🧪

Test	Results
Failed	0
Passed	167
Skipped	3
Total	170

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

[fearful-symmetry]

/test

[cmacknz]

We'll want to update the dependency in the branch for elastic/beats#32153, the tests there reproduce the problem.