[Improvement] Reporting the metrics of the clientPool in hive, iceberg, and paimon

### What would you like to be improved?

We are using the gravitino iceberg rest server in Xiaomi to manage iceberg metadata.

Currently, the connection pools of the iceberg backend catalog have no observable metrics.
we have collected the related metrics, such as the number of active connections, number of idle connections, and the size of the connection pool in iceberg hiveClientPool, and reported to prometheus.

In addition to this, I think it is also possible to collect metrics of hive and Paimon-related connection pools to improve the observability of the service.

### How should we improve?

_No response_

Start by locating the iceberg hiveClientPool metrics and the existing Prometheus reporting path mentioned in the issue, then compare the corresponding Hive and Paimon connection-pool entry points. Done means the active connections, idle connections, and pool size are observable for the Hive and Paimon pools as well, with tests or verification covering the reported metrics.

Written by the indexing model from the issue text.