Summary

Migrates the messaging.queue.depth metric from a synchronous gauge — recorded imperatively on the message hot path — to an observable gauge sampled by the telemetry SDK at each collection interval.

Previously the depth was recorded via Gauge::record() once at worker start and again in the finally after every processed message. That coupled the sampling cadence to throughput:

• Idle / stuck queue → stale metric. If jobs stop flowing (all failing, no healthy consumer), record() never fires and the gauge goes stale exactly when it matters most.
• High throughput → redundant work. A Redis listSize round-trip ran on the hot path for every message.

An observable gauge fixes both: the SDK invokes the registered observe() callback on a fixed export cadence, decoupled from message processing, and uses last-value semantics.

Changes

• composer.json — bump utopia-php/telemetry 0.2.* → 0.4.* (adds createObservableGauge).
• src/Queue/Server.php — swap Gauge/createGauge for ObservableGauge/createObservableGauge; register an observe() callback that reads getQueueSize() and reports it with the same attributes (messaging.destination.name/namespace) and the same Publisher-check and try/catch guards. Remove recordQueueDepth() and both call sites.
• tests/.../ServerTelemetryTest.php — update to the pull model: assert against $telemetry->observableGauges and invoke the registered callback via a collectObservations() helper (one invocation = one collection cycle).

Not covered here

This does not by itself fix the Grafana double-counting on the dashboard — that's a fan-in problem (N workers/pods each reporting the same shared Redis depth under identical attributes), which needs the dashboard query to max/avg across series rather than sum(). The gauge also still only samples the pending queue, not the failed queue. Both are tracked as follow-ups.

Test plan

• composer run test (ServerTelemetryTest 3/3 pass)
• composer run check (phpstan clean, --memory-limit=512M)
• composer run lint (pint passed)

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