Make {system,process}.cpu.utilization opt-in

[ChrsMark]

Fixes #1130

Changes

This PR changes the requirement level of the system.cpu.utilization and process.cpu.utilization from recommended to optional according to the decision at #1130.
The main argument here is that metrics which can be derived from other metrics should be optional.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.