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

[bharos]

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.