[Feature] Introduce metrics about the busyness of compaction thread

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/incubator-paimon/issues) and found nothing similar.

### Motivation

This metric monitors how busy the writer is, and if it is too busy, it can alert the user to add resources in advance.

### Solution

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!