Add warning alert for too high distributor GC CPU utilization

[aknuds1]

What this PR does

Add a warning severity alert for too high distributor garbage collection CPU utilization. The motivation is to be alerted if GOMEMLIMIT causes distributors to garbage collect too often.

Additionally, enable three CPU runtime metrics required by the alerts:

• /cpu/classes/gc/total:cpu-seconds
• /cpu/classes/total:cpu-seconds
• /cpu/classes/idle:cpu-seconds

Which issue(s) this PR fixes or relates to

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• about-versioning.md updated with experimental features.

[github-actions]

💻 Deploy preview deleted.

[pracucci]

Reviewed doing pair programming. LGTM!

[grafanabot]

The backport to r314 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-10641-to-r314 origin/r314
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 1ce9dff5f306b906eff412bf9b5f62c6ae9dd102
# Push it to GitHub
git push --set-upstream origin backport-10641-to-r314
git switch main
# Remove the local backport branch
git branch -D backport-10641-to-r314

Then, create a pull request where the base branch is r314 and the compare/head branch is backport-10641-to-r314.