Context

The Aggregation executor is the universal fallback and the entire lookup path when the IFR flag is
off — the executor running in production today. Instrumenting it first lands a working end-to-end
metric on the safest path.

Scope

• Add a SingleDocumentLookupStatsRecorder constructor parameter to AggregationSingleDocumentLookupExecutor; store it.
• In performLookup, call recordFound / recordNotFound with the measured latency. The Aggregation executor always handles the lookup (it is the universal fallback and never returns kNotHandled), so it never records notHandled.
• Update the stage-build factory (makeUpdateLookupStage site) to inject the updateLookupAggregationStats cell.

Acceptance

With the IFR flag OFF, a stream with updates increments changeStreams.updateLookup.aggregation.found / notFound and the latency histogram.

Tests (definition of done)

Create the shared replset jstest jstests/change_streams/change_stream_metrics_update_lookup.js:

• tagged assumes_against_mongod_not_mongos + assumes_read_preference_unchanged + assumes_no_implicit_cursor_exhaustion (per the change_stream_metrics_* family).
• baseline -> operate -> delta on serverStatus.
• flag-off branch: assert the aggregation cell increments.
• topology-agnostic invariants (found + notFound + notHandled reconcile; monotonic).
• compute the expected cell as expectedEngine(flag, changeStreamPassthroughType()) so the Express and SBE wiring tickets can add their branches without restructuring.

Dependencies

Depends on the metrics infrastructure ticket, and AggregationSingleDocumentLookupExecutor (SERVER-128409).