Query-snapshots 1s lock-timeout yields (error 1222) are counted as collection errors, so one benign yield paints a Critical health day #1805

Observed (a production field instance, post-upgrade soak)

Rare, sporadic collector queries hitting the 1-second lock timeout. The timeout itself is the DESIGN WORKING: exactly one collector carries SET LOCK_TIMEOUT 1000 (QuerySnapshotsCollector, both query variants), and yielding after 1s instead of joining a blocking chain is the monitor keeping its never-be-a-blocker promise. The data cost of a hit is near zero: one point-in-time snapshot sweep skipped; the next sweep sees current state; nothing cumulative or watermarked loses anything.

The defect is the CLASSIFICATION, not the yield

No code anywhere handles SQL Server error 1222 specially (verified: zero matches across the collectors, the service, and Common), so the yield surfaces as a generic collection error: an error row in collection_log, counted into collector failure rates — and the daily-health banding treats ONE collection error, alone, as a CRITICAL day (pinned by DailyHealthBandTests.CriticalTriggers_EachAloneIsCritical(collErrors: 1)). Each benign 1-second yield therefore paints a Critical day on the health calendar, which draws repeated operator/agent attention to a non-event. That amplification — not the missed snapshot — is the cost.

Reframe worth keeping

A 1222 on the snapshot collector is evidence about the TARGET's lock contention — signal about the monitored server, currently mislabeled as a monitoring failure.

Fix sketch (classification-only; the collection behavior is correct and stays)

• Catch error 1222 on the lock-timeout-guarded collector and record it as a deliberate YIELD: its own log shape and counter, distinct from collection errors.
• Exclude yields from the collection-error count that feeds the Critical band.
• Sustained clustering (many yields on one server/window) can still surface — as target-contention signal, not monitor failure. Thresholds conservative; defaults over knobs.
• Do NOT raise the timeout, add hot retries, or extend the guard to other collectors speculatively.

Severity

Low; nothing gates on it. Post-release queue.