[Feature] The maximum watermark value of the current commit should be reported by `CommitterMetrics`

### Search before asking

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

### Motivation

I write paimon through the flink. After commit, I hope to report the watermark as an indicator, so that I can know the freshness of the current paimon table instead of obtaining it through the system table. That query is too slow. Through flink Once the metric report comes out, it will be easier for me to combine it with a third-party monitoring system.

### Solution

Report the watermark when committing

### Anything else?

_No response_

### Are you willing to submit a PR?

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

Start by locating CommitterMetrics and the commit path used by Flink. Trace how the current commit watermark is available, then determine how the maximum value should be exposed as a metric and add coverage showing that monitoring consumers receive it.

Written by the indexing model from the issue text.