[chore] Move system metric attributes to the registry

[gregkalapos]

Changes

Moves system.* attributes to the registry from system metrics.

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.

[ChrsMark]

Not sure if the state of this PR is "ready for review" already but my comment seems to be still unresolved?

I think we have concluded on having the registry in a signal generic form. In this I left some suggestions to remove any mention to "Metrics" from the registry's attributes.

And we will still need to override the system.cpu.state within the system metrics spec.

Let me know @gregkalapos if I miss anything or there is anything that is unclear ;).

[gregkalapos]

Not sure if the state of this PR is "ready for review" already but my #867 (comment) seems to be still unresolved?

Sorry for the confusion - let's discussed it in that thread and depending on what we agree on, I'll move on with the suggestions.

[ChrsMark]

Thank's @gregkalapos! Looks good from my side.