Summary

Add aggregated change-stream cursor-level lag gauges to $currentOp on mongoS.

Background

Design doc: "Design: Extend ChangeStream Metrics", section "Decision: Cursor-Level Change Stream
Metrics / Lag". These are aggregated from shard responses on every batch, not computed locally on
the router.

Scope of Work

• Add the following gauges, aggregated from shard responses on every batch:
  • changeStreams.oplog.earliestTime.lastBatch.min / .max
  • changeStreams.oplog.latestTime.lastBatch.min / .max
• Surface these in the slow-query log, per the design doc.

  Acceptance Criteria

• All five fields appear under $currentOp for active change-stream cursors on mongoS and
  update once per batch based on shard responses.
• Fields appear in the slow-query log for change-stream cursors on mongoS.
• Unit/integration test coverage, including multi-shard aggregation cases.

  Related Files

• src/mongo/s/query/exec/cluster_client_cursor_impl.cpp
• src/mongo/s/query/exec/cluster_cursor_manager.cpp