Add user data to monitoring setup

[fearful-symmetry]

What does this PR do?

This is a simple PR that migrates the code that adds user data to the monitoring registry from beats to here. The original code is in libbeat/cmd/instance/beat.go, and needs to be used in the shipper in order to bring the shipper up to feature parity with the beats in terms of monitoring. Just copy-and-pasting the user metrics is a bit ugly, so I figured I'd be better off migrating the metrics.

Why is it important?

We want to reduce code duplication across beats.

Checklist

• [x] My code follows the style guidelines of this project
• [x] 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]

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#### Build stats * Start Time: 2023-03-27T18:21:22.998+0000 * Duration: 10 min 47 sec #### Test stats :test_tube: | Test | Results | | ------------ | :-----------------------------: | | Failed | 0 | | Passed | 188 | | Skipped | 7 | | Total | 195 |

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