feat: metrics for outboxed services

[PDT42]

Add collection of queue metrics - unless the feature is disabled - by registering a done handler, which will trigger metrics collection. Metrics collection for a queued service will start once the service in question is found to be the target of a queued message for the first time.

Correct collection of the cold_entries metric depends on the maxAttempts queue configuration being available from the queued service, which is not yet the case for cds^8. For cds^8 maxAttempts is currently being defaulted (to 20).

Counts of incoming_messages and outgoing_messages are currently being collected per tenant per app instance and AFAIK should be aggregated per tenant by the collector.

[sjvans]

looks good! i want to go over the lingo "outbox" vs "queue" one more time. plus, we should also optimize local export to console like we do for db pool metrics before we release. otherwise we will spam the console!

[sjvans]

🚀