[#8912] feat(iceberg-rest): Expose Iceberg client metrics through Gravitino MetricsSystem

[bharos]

What changes were proposed in this pull request?

This PR adds observability for Iceberg client operations by bridging Iceberg's metrics reporting to Gravitino's MetricsSystem.

Key Changes:

IcebergClientMetricsSource: New metrics source with iceberg-client namespace (separate from iceberg-rest-server HTTP metrics)
IcebergRestMetricsStore: Implements MetricsStore to parse and record Iceberg commit/scan metrics using Iceberg's public APIs
Configuration: Enable with metricsStore = rest

Why are the changes needed?

Metrics sent to /v1/{prefix}/namespaces/{namespace}/tables/{table}/metrics are silently dropped when using dummy store. This PR enables monitoring of:
Iceberg table operations (commits, scans)
Data file operations (added/removed files, sizes)
Query performance metrics sent through the metrics API

Fix: #(issue)

Does this PR introduce any user-facing change?

Yes, new configuration and metrics:

# Server configuration
gravitino.iceberg-rest.metricsStore = rest

# Client configuration (Spark)
spark.sql.catalog.<catalog-name>.rest-metrics-impl = org.apache.iceberg.rest.RESTMetricsReporter

Exposed metrics (under iceberg-client namespace): commit reports, scan reports, data files added/removed, file sizes, scan/commit durations, and 27+ additional metrics.

How was this patch tested?

• Unit tests:

./gradlew :iceberg:iceberg-rest-server:test --tests TestIcebergRestMetricsStore

• Production verification: Deployed to K8s with Spark SQL workload, confirmed 32 metrics tracked correctly

 curl -s http://localhost:9001/metrics | jq '.histograms | with_entries(select(.key | startswith("iceberg-client")))'
{
  "iceberg-client.iceberg.total-duration": {
    "count": 3,
    "max": 0,
    "mean": 0,
    "min": 0,
    "p50": 0,
    "p75": 0,
    "p95": 0,
    "p98": 0,
    "p99": 0,
    "p999": 0,
    "stddev": 0
  },
  "iceberg-client.iceberg.total-planning-duration": {
    "count": 9,
    "max": 0,
    "mean": 0,
    "min": 0,
    "p50": 0,
    "p75": 0,
    "p95": 0,
    "p98": 0,
    "p99": 0,
    "p999": 0,
    "stddev": 0
  }
}

curl -s http://localhost:9001/metrics | jq '.counters | with_entries(select(.key | startswith("iceberg-client")))'
{
  "iceberg-client.iceberg.added-data-files": {
    "count": 1
  },
  "iceberg-client.iceberg.added-files-size-bytes": {
    "count": 960
  },
  "iceberg-client.iceberg.added-records": {
    "count": 1
  },
  "iceberg-client.iceberg.attempts": {
    "count": 3
  },
  "iceberg-client.iceberg.dvs": {
    "count": 0
  },
  "iceberg-client.iceberg.equality-delete-files": {
    "count": 0
  },
  "iceberg-client.iceberg.indexed-delete-files": {
    "count": 0
  },
  "iceberg-client.iceberg.positional-delete-files": {
    "count": 0
  },
  "iceberg-client.iceberg.removed-data-files": {
    "count": 1
  },
  "iceberg-client.iceberg.removed-files-size-bytes": {
    "count": 923
  },
  "iceberg-client.iceberg.removed-records": {
    "count": 1
  },
  "iceberg-client.iceberg.reports.commit": {
    "count": 3
  },
  "iceberg-client.iceberg.reports.scan": {
    "count": 9
  },
  "iceberg-client.iceberg.result-data-files": {
    "count": 5
  },
  "iceberg-client.iceberg.result-delete-files": {
    "count": 0
  },
  "iceberg-client.iceberg.scanned-data-manifests": {
    "count": 5
  },
  "iceberg-client.iceberg.scanned-delete-manifests": {
    "count": 0
  },
  "iceberg-client.iceberg.skipped-data-files": {
    "count": 0
  },
  "iceberg-client.iceberg.skipped-data-manifests": {
    "count": 2
  },
  "iceberg-client.iceberg.skipped-delete-files": {
    "count": 0
  },
  "iceberg-client.iceberg.skipped-delete-manifests": {
    "count": 0
  },
  "iceberg-client.iceberg.total-data-files": {
    "count": 1
  },
  "iceberg-client.iceberg.total-data-manifests": {
    "count": 7
  },
  "iceberg-client.iceberg.total-delete-file-size-in-bytes": {
    "count": 0
  },
  "iceberg-client.iceberg.total-delete-files": {
    "count": 0
  },
  "iceberg-client.iceberg.total-delete-manifests": {
    "count": 0
  },
  "iceberg-client.iceberg.total-equality-deletes": {
    "count": 0
  },
  "iceberg-client.iceberg.total-file-size-in-bytes": {
    "count": 4615
  },
  "iceberg-client.iceberg.total-files-size-bytes": {
    "count": 960
  },
  "iceberg-client.iceberg.total-positional-deletes": {
    "count": 0
  },
  "iceberg-client.iceberg.total-records": {
    "count": 1
  }
}