[Improvement] Metrics sent to Iceberg Metrics REST API currently dropped. Expose Iceberg client metrics through Gravitino MetricsSystem

### What would you like to be improved?

Currently, when the Iceberg REST server receives metrics from clients via the
`/v1/{prefix}/namespaces/{namespace}/tables/{table}/metrics` endpoint, they are
silently dropped if metricsStore is set to "dummy" (the default). This provides
no observability into:

- Iceberg table operations (commits, scans)
- Data file operations (added/removed files, sizes)
- Query performance and patterns

We need a way to expose these metrics for monitoring and debugging.

### How should we improve?

1. Add IcebergRestMetricsStore that implements Iceberg's MetricsStore interface
and bridges metrics to Gravitino's MetricsSystem

2. Add IcebergClientMetricsSource with "iceberg-client" namespace (separate from
"iceberg-rest-server" HTTP metrics to avoid collision)

3. Parse commit and scan reports using Iceberg's public APIs (CommitReportParser,
ScanReportParser)

4. Expose metrics via the existing /metrics endpoint under iceberg-client namespace

5. Allow configuration via: gravitino.iceberg-rest.metricsStore = rest

This will enable production observability of Iceberg operations through Gravitino's
existing metrics infrastructure.