0293 — The abandoned-goroutine registry is a mutex-guarded map keyed by (MessageId, Attempt), kept as plain in-process state

Context. Timed-out consumerFunc goroutines cannot be killed, only tracked. The registry needed a key, a synchronization choice, and a decision on how formal to make the metrics before any metrics-library commitment existed.

Decision. ConsumerMetrics.AbandonedRoutines is a map[abandonedKey]time.Time keyed by (MessageId, Attempt) — a message’s first and second attempts can each leave a live abandoned goroutine, so message id alone would let one entry overwrite the other. It is guarded by a sync.Mutex, with TrackingTotal() (atomic counter), CurrentTotal() (gauge from map length), and ReclaimLatency() (average over a self-locking ConcurrentBoundedRingBuffer[time.Duration], capacity 256, Values() returning a defensive copy). The shape is deliberately informal — no metrics-library commitment yet; wiring these already-built numbers into a pluggable interface is deferred until one is chosen.

Consequences. Labs can assert against a live gauge immediately; a later metrics layer inherits working numbers instead of a one-off shape invented early. Rejected: sync.Map — the access pattern is one Add/Remove/CurrentTotal call each per event, not enough concurrent contention to justify its striped-lock complexity. Rejected: keying by message alone — under-keys when multiple attempts of the same message are simultaneously abandoned.