Add outbox metrics

[taharafique]

Summary

Requirements and ADR for OpenTelemetry metrics on the Outbox (spec 0037). Docs only, no code changes yet.

Today the Outbox's only signal is trace spans. The add path often has a null span and production sampling drops the rest, so we have no aggregate view of Outbox volume or lag.

Proposal (ADR 0072)

OutboxProducerMediator records to three instruments directly, behind a new IAmABrighterOutboxMeter. Nothing is derived from traces, so metrics record even when tracing is off or unsampled.

Instrument	Type	Attributes
paramore.brighter.outbox.added.messages	Counter<long>	messaging.destination.name
paramore.brighter.outbox.cleared.messages	Counter<long>	+ paramore.brighter.outbox.clear_source (explicit / sweeper / unknown)
paramore.brighter.outbox.publish.duration	Histogram<double> (s)	same as cleared

• Clear source is passed down to the private dispatch methods as an OutboxClearSource parameter.
• For publish-confirmation producers, creation time is held in a bounded, TTL-evicted pending-clear map until the broker confirms.
• No public API break, no changes to Outbox implementations, no schema change.

Open questions for maintainers

• .duration vs .latency for the histogram name
• Counter<long> vs Counter<int>, to match MessagingMeter

Files

• specs/0037-outbox-metrics/requirements.md
• docs/adr/0072-outbox-metrics.md

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[iancooper]

Thanks for this @taharafique It looks good to me, go ahead

[iancooper]

@taharafique My main question is whether I always need an OutboxMeter. Typically, we would make this optional, so passing null means you have no meter and can return quickly from any meter-recording calls.

Typically, we would configure this as part of our setup of Telemetry and meters; see: src/Paramore.Brighter.Extensions.OpenTelemetry/OpenTelemetryBuilderExtensions.cs, and that ought to be where we enable it

This would also mean that you don't need to include it in BuildOutBoxProducerMediator because you would built it as part of the set up of our telemetry.

[iancooper]

Thanks @taharafique

[iancooper]

Thanks @taharafique