feat(compact): Add Pushgateway metrics support for one-shot runs

[elohmeier]

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

Changes

This change introduces the ability for the thanos compact command to push its metrics to a Prometheus Pushgateway when running in one-shot mode (without the --wait flag).

This is particularly useful for monitoring the compactor when it is run as a periodic batch job, such as a cron job.

The following changes have been made:

• Refactored progress metric calculation to run in one-shot mode.
• Added --push-gateway.url and --push-gateway.job flags to configure the Pushgateway endpoint and job name.
• Implemented the logic to push all registered metrics upon successful completion of a one-shot compaction run.

Verification

I've tested the metrics availability in the --wait mode and with a Pushgateway.

[elohmeier]

Added CHANGELOG entry.

[elohmeier]

Docs CI failure is unrelated, I've pushed a fix here: https://github.com/thanos-io/thanos/pull/8494