[Feature] Add lastCommittedSnapshotId commit metric and document lastScannedSnapshotId scan metric

### Search before asking

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

### Motivation

Paimon's commit metrics currently provide detailed information about file counts, sizes, durations, and compaction stats, but they do not expose the snapshot ID produced by each commit. Similarly, the lastScannedSnapshotId scan metric exists in code but is missing from the metrics documentation. Adding these improves observability for operators monitoring streaming ingestion pipelines.

### Solution

Add lastCommittedSnapshotId gauge to commit metrics. Document lastScannedSnapshotId and lastCommittedSnapshotId in metrics reference

### Anything else?

_No response_

### Are you willing to submit a PR?

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

Inspect the existing commit metrics implementation and the metrics reference documentation, then locate the existing lastScannedSnapshotId scan metric. Add the lastCommittedSnapshotId gauge and document both metrics; done means the commit metric exposes the produced snapshot ID and the reference covers both names.

Written by the indexing model from the issue text.