Merge *.cpu.state attributes to a common cpu.mode attribute

[ChrsMark]

Fixes #840

Changes

This PR introduces a common cpu.state cpu.mode attribute which is a converged attribute from the system.cpu.state, process.cpu.state and container.cpu.state.

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]

LGTM

I think you need to re-generate the make generate-gh-issue-templates though

[ChrsMark]

Thank's @joaopgrassi. For some reason the make generate-gh-issue-templates does not change anything 🤔 .

[ChrsMark]

@open-telemetry/semconv-system-approvers could you take a look and specifically into the #1026 (comment)? Thank's.

[ChrsMark]

Changed to cpu.mode @lmolkova @trisch-me @open-telemetry/semconv-system-approvers feel free to take another look :)

[trisch-me]

You need to regenerate registry to update md files to fix the issue

[lmolkova]

Looks good! Just left a few minor comments.