Add cluster_id tag for TabletServer client metrics

### Search before asking

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

### Description

Currently, the netty client metrics of TabletServer have no cluster_id tag. This makes it difficult to distinguish which cluster sent the request.

### Willingness to contribute

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

Start by locating the TabletServer netty client metrics entry point and inspect how metric tags are assembled. Trace where the cluster identity is available, then verify that the resulting metrics include a cluster_id tag so requests from different clusters can be distinguished.

Written by the indexing model from the issue text.