Summary

Add metrics and structured logs for collection hash repair so operators can determine whether repairs are being requested, making progress, successfully incorporated into replicated metadata, or repeatedly failing.

Proposed Metrics

• Repair requests and outcomes: counters for requests, committed repair oplog entries, failures, and no-ops. Distinguish no-op reasons such as collection no longer existing or empty repair metadata.
• Repair mode: distinguish hash replacement (hSet) from XOR adjustment (hDiff), and hash repairs from size/count-only repairs. hDiff cannot restore an absent hash.
• Checkpoint incorporation: count repairs incorporated into metadata checkpoints and measure time from committed repair entry to incorporation (this depends on SERVER-137710)

Structured Logs

Emit repair lifecycle events with collection UUID, repair mode, outcome, wall-clock event time, repair oplog timestamp, and checkpoint valid-as-of boundary when available.