mixin: Add block deletion panels for compactor dashboard

[Sayrus]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

This PR adds a new row to the Thanos Compact Dashboard. This row contains:

• The rate at which blocks are deleted from S3: This is useful as it avoids reading logs or manually querying the metric. In 0.15, the deletion is done after downsampling so it allows us to monitor whether it is actually called or not. In 0.16, this process is done concurrently thanks to cmd: compact: clean partial / marked blocks concurrently #3115 . This graph then allows us to monitor if the async context is doing its job.
• The error rate when trying to delete from S3
• The rate at which blocks are marked for deletion, not only from GC but also from retention policy

Verification

Our compactor isn't deleting anything at the moment so the graphs are empty.

Notes
Metrics will need to be renamed following #3309 as they use the thanos_compact_* format.

[kakkoyun]

Thanks for the PR. I can see the use case.
Let's wait for the renaming PR to be merged.

[metalmatze]

Renaming PRs should be merged.

[Sayrus]

Updated for new names 👌

[kakkoyun]

@Sayrus Looks good so far. You need to run make examples and commit changes to make CI happy.

[kakkoyun]

nit: Let's be super clear on what we actually add in the changelog.

Then we can merge.