[Build][Docs] Generate list of metrics in docs from source code

### Search before asking

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

### Motivation

Generate Fluss-specific metrics [in the documentation](https://alibaba.github.io/fluss-docs/docs/maintenance/observability/monitor-metrics/#metrics-list) from the source code (`src/main/java/com/alibaba/fluss/metrics/MetricNames.java`).

### Solution

_No response_

### Anything else?

_No response_

### Willingness to contribute

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

Start with src/main/java/com/alibaba/fluss/metrics/MetricNames.java and compare its metrics with the documentation's metrics list at docs/maintenance/observability/monitor-metrics/#metrics-list. Determine how the list can be generated from the source, then verify that the documentation reflects the source-defined metrics and remains suitable for the existing docs.

Written by the indexing model from the issue text.