Add block compaction delay metric

[jhalterman]

What this PR does

Adds a new summary metric to compactor, cortex_compactor_block_compaction_delay_seconds, which is the time between a block being created/updated and it being successfully compacted. The motivation for this metric is wanting to understand when there are delays in compacting blocks, such as from compactors being or restarting, that may indirectly cause non-compacted blocks to be queried via store-gateways.

Which issue(s) this PR fixes or relates to

Fixes #

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.

[pstibrany]

As a first step, I'd prefer to avoid adding new field to struct representing meta.json.

[pstibrany]

Thank you. Left some minor comments.

[pstibrany]

Thank you. (We may want to mention new metric in the changelog too)

[grafanabot]

The backport to r282 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-7635-to-r282 origin/r282
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 96952cf7bbfbdbf5fa77f22032e834a12a8ac236
# Push it to GitHub
git push --set-upstream origin backport-7635-to-r282
git switch main
# Remove the local backport branch
git branch -D backport-7635-to-r282

Then, create a pull request where the base branch is r282 and the compare/head branch is backport-7635-to-r282.