Downstream Changes Summary

Project context
---------------
With featureFlagChangeStreamOptimizedUpdateLookup on, collection-level change streams resolve
updateLookup fullDocuments on the SBE executor: it compiles a parameterized {_id: <value>} plan
once and reuses it across events, with the collection acquisition cached across the batch window.
SBE may decline a lookup (unsupported _id shape, collation, ...); declines fall through to the
routed Aggregation executor (see SERVER-129513).

What this ticket adds
---------------------
New serverStatus metric names under metrics.changeStreams.updateLookup.sbe, populated from the
SBE executor, completing the per-engine set (express, sbe, aggregation). Reading the three cells
together shows how lookups distribute across engines and whether engine changes shift work to
the fallback.

Metric semantics
----------------
Same outcome model as the other engines: found / notFound / notHandled (decline, not an error) -
latencyMicros (histogram, microseconds), recorded only for executed lookups.
Scope note: SBE latency covers the WHOLE eligibility run - including any stale-config routing
refresh and retry - not just the terminal index attempt, because that is what the caller
actually waited for.

No new commands, options, response fields, or wire-protocol changes; no change to lookup results.

Description of Linked Ticket

Context

SBE is the intended primary local executor for collection-level update-lookup streams (flag on). SBE
support has not landed yet and is contingent on the SBE feasibility go/no-go, so this ticket is
future — blocked until the SBE executor and the collection-level factory switch exist. The
fall-through SBE can trigger (kNotHandled -> Aggregation) is verified separately in the sharded
fall-through ticket.

Scope

• Add the updateLookupSbeStats accessor (registers the cell — not in the infrastructure ticket).
• Add the recorder constructor parameter and the record* calls in SbeSingleDocumentLookupExecutor.performLookup.
• The collection-level updateLookup factory injects the updateLookupSbeStats cell.
• planBuilds-per-cursor stays a feasibility-branch diagnostic — do NOT ship a serverStatus metric.
• Pre-image-on-SBE is a separate path; its addPreImage recorder wiring is the pre-image telemetry ticket, not this one.

Acceptance

A collection-level stream with updates increments changeStreams.updateLookup.sbe.found / notFound + latency.

Tests

Extend the shared metrics jstest with the (flag on, collection) branch: when the flag is on and
changeStreamPassthroughType() is kCollection, expect the sbe cell. No new file.

Dependencies

Depends on the metrics infrastructure ticket, the SBE executor and the collection-level factory switch
(not yet built), and the Aggregation wiring ticket (shares the jstest). Future — blocked until SBE
lands, contingent on the SBE feasibility go/no-go.