Add GetOneRootEvent method to process handler

[fearful-symmetry]

What does this PR do?

This is part of https://github.com/elastic/elastic-agent/issues/4083

This adds a GetOneRootEvent(). method to ProcStats we need this for https://github.com/elastic/elastic-agent/issues/4083 , as that issue requires metricbeat's system/process metricset to monitor a specific PID, which it can't do now. This will have to be followed up with a change to beat itself to rope this in. This also cleans up a bit of code and adds some docs.

Why is it important?

needed for https://github.com/elastic/elastic-agent/issues/4083

Checklist

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