[Metrics] Deadline Proximity histogram (time-to-deadline of queued items)

[shimib]

Split out of #217 (comprehensive metrics), which is now closed with the core suite delivered. This is a post-v0.9.0 fast-follow.

What

A histogram of deadline - now() across items currently in the queue, to spot when a large batch of work is about to expire simultaneously (SLO early warning).

• Metric: async_deadline_proximity_millis (histogram), labeled by pool/queue.

Approach

Piggyback the existing per-queue backlog poller (the ZCard loop + ticker that powers async_broker_backlog): add a bucketed ZRANGEBYSCORE/ZCOUNT read of the SortedSet scores (deadlines) and observe deadline - now() per sampled item.

Caveats

• redis-sortedset only — Pub/Sub can't peek at un-consumed messages, so this metric is not feasible for the redis-pubsub broker (document the limitation).
• Sampled on the poller interval (a snapshot per tick), not continuous.