Align system.cpu.state's definition with this of process.cpu.state

[ChrsMark]

Fixes #563

Changes

This PR aligns the definition of the system.cpu.state's with this of process.cpu.state. Specifically it adds the same constraint of process.cpu.states as mentioned at #563 (comment).

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.

[joaopgrassi]

Shouldn't we instead move the attribute to the registry, and simply re-use it in process/system metrics? There's also this which is related #681

[ChrsMark]

Thank's @joaopgrassi for pointing to this. Check #765 (comment).