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

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