feat(metrics): Instrument commits and emit CommitReport ( #1236)

[tanmayrauth]

What

Emits a spec-compliant CommitReport from the commit path on successful completion:

• doCommit's retry loop now records total-duration and attempts.
• The committed snapshot's summary is mapped into a CommitReport under Java's commit-report metric names. iceberg-go's summary keys differ from those names, so the mapping translates them (added-files-size →
  added-files-size-bytes, deleted-data-files → removed-data-files, added-position-deletes → added-positional-deletes, …) so the wire format matches Java and dashboards line up across implementations.
• Metrics with no iceberg-go summary key (DVs, manifest counts) are omitted rather than reported as zero, mirroring Java's counterFrom.

Behavior notes

• No behavior change for existing users: emission is guarded by metrics.IsNop, so under the default no-op reporter it's a pure no-op (no report assembled). Report failures can never fail a commit.
• Metadata-only commits emit nothing: a property/schema-only change produces no snapshot, so reporting the unchanged branch head would misattribute a prior snapshot's metrics. Skipped via
  commitAddedSnapshot.
• Non-default branches resolve the just-committed head via SnapshotByName(branch) rather than the table's current snapshot.

Deferred

CommitReport.Metadata (Java's engine/commit-context map) is left unset with a TODO — iceberg-go doesn't thread that context into the commit path yet; a follow-up will populate it.

Testing

• Unit tests for buildCommitReport: name-translated mappings, nil snapshot, absent/unparseable summary values omitted.
• Integration tests: an append emits a CommitReport with correct operation/attempts/records; a metadata-only commit emits none.
• go build, go vet, and full table/metrics suites pass.

Realted to #1236