Add some metrics for CompletedSnapshotStoreManager to monitor the remote kv Snapshot

### Search before asking

- [x] I searched in the [issues](https://github.com/alibaba/fluss/issues) and found nothing similar.

### Motivation

During the testing, I found that the `CompletedSnapshotStoreManager` lacks some key metrics, which makes it difficult to locate issues. These metrics, for example, `the avg time to delete a remote kv Snapshot`, `the number of remote kv Snapshots waiting to be deleted`, and `the rate of create or deleted remote kv snapshots per second` are not available now.

### Solution

Add more metrics.

### Anything else?

_No response_

### Willingness to contribute

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