Report DataFusion operator metrics in IcebergTableScan

[toutane]

Is your feature request related to a problem or challenge?

IcebergTableScan does not implement ExecutionPlan::metrics(), so EXPLAIN ANALYZE shows an empty metrics=[] on the scan line while every other operator in the plan reports its cost.

This makes query profiling against Iceberg tables blind on the scan, and prevents quantifying optimizations that touch the scan.

Reproducer: EXPLAIN ANALYZE SELECT * FROM t WHERE foo = 1:

CoalesceBatchesExec: ..., metrics=[output_rows=1, elapsed_compute=19µs]
  FilterExec: foo@0 = 1, metrics=[output_rows=1, elapsed_compute=90µs]
    RepartitionExec: ..., metrics=[fetch_time=4.4ms, ...]
      IcebergTableScan predicate:[foo = 1], metrics=[]        <-- empty

Describe the solution you'd like

Wire BaselineMetrics + ExecutionPlanMetricsSet into IcebergTableScan, same pattern as ParquetExec.

Scope limited to the two default metrics (elapsed_compute, output_rows). bytes_scanned is a natural follow-up once #2349 lands: its ScanMetrics counter can be exposed as a custom DataFusion metric in a separate PR.

Willingness to contribute

I can contribute to this feature independently