Compact: Retention progress metrics

[yeya24]

Is your proposal related to a problem?

Because retention happens after compaction and downsampling, so it might get huge retention backlog to do in the object storage. We also got the same problem mentioned in #4406.

It would be great to have a way to tell how many blocks are beyond their retention period but don't get deleted.

Describe the solution you'd like

We have introduced the progress calculator to get compaction and downsampling progress as metrics in #4801.

We can implement another RetentionProgressCalculator to do this work.