Context

Change streams enrich update events with a post-image via a single-document lookup performed by one
of three executors (Aggregation / Express / SBE). We want per-(consumer x engine) metrics so
operators can see lookup volume, outcomes, and latency, and so tests can assert which executor
handled a lookup. Metrics are process-global singletons; an executor is per-cursor and ephemeral, so
it borrows a recorder rather than owning metrics.

Design: SPM-4535 plan, "Observability" section.

Scope

• In single_document_lookup_stats.h / .cpp (new, under src/mongo/db/exec/single_doc_lookup/): the struct SingleDocumentLookupStats (references to one cell's 4 OTEL instruments — Counter found / notFound / notHandled + a latency Histogram, microseconds, buckets ~50us..100ms) and the concrete, non-virtual SingleDocumentLookupStatsRecorder (holds a const reference to one cell; methods recordFound / recordNotFound (Microseconds) and recordNotHandled()).
• In change_stream_metrics_util.h: a create helper per cell, mirroring the existing createCursors* helpers, building the cell's instruments with serverStatusOptions.dottedPath = "changeStreams.<consumer>.<engine>.<stat>". Each helper's metric name is a one-line MetricNames entry in otel/metrics/metric_names.h — the standard two-part OTEL addition the cursor metrics already use.
• Add the updateLookup express and aggregation cells (updateLookupExpress, updateLookupAggregation).
• No executor records yet — no behaviour change.

Acceptance

• serverStatus().metrics.changeStreams.<consumer>.<engine>.<stat> appears (zeroed) for every registered cell at startup, on a mongod.
• The recorder is a concrete, non-virtual type; no interface or mock is introduced — metrics are verified by reading the real OTEL instruments via test utilities (OtelMetricsCapturer).
• No classic MetricBuilder metric is added (OTEL only).

Tests

C++ unit test single_document_lookup_stats_test.cpp: construct a recorder over a real cell, call each
record method, and read the instruments back with OtelMetricsCapturer (otel/metrics/metrics_test_util.h,
the cursor_manager_test.cpp pattern). Assert the counters, and that latency is recorded on
found/not-found but not on not-handled.