Context

motel emits CONSUMER for async children (pkg/synth/engine.go:449-455) but never emits PRODUCER, the fifth OTel span kind.

What the OTel spec says

Per the SpanKind spec and messaging semconv:

• PRODUCER → the initiator of an asynchronous request (e.g. enqueuing a message); it typically ends before the corresponding CONSUMER span starts.
• CONSUMER → the child of an asynchronous PRODUCER request (e.g. processing a dequeued message).

Today motel models only the consumer side of async work: an async: true call produces a CONSUMER child, but there is no explicit PRODUCER span representing the enqueue/publish operation. A realistic messaging trace usually has both: a PRODUCER span on the publishing service and a (often linked, separate-trace) CONSUMER span on the processing service.

Proposed work

• Allow an async send to be modelled with an explicit PRODUCER span for the enqueue/publish step, distinct from the CONSUMER span that processes it
• Interplay with existing span-link support: producer and consumer commonly live in different traces joined by a span link (see docs/examples/span-links.yaml) — make sure the two compose
• Decide how this is expressed in the DSL (e.g. a producer/consumer pairing on a messaging call) without breaking the current async/CONSUMER behaviour
• Tests asserting PRODUCER on the publish side and CONSUMER on the process side

Acceptance criteria

• Topology can produce explicit PRODUCER spans for async producers
• Existing CONSUMER behaviour preserved
• Producer/consumer pairing composes with span links
• Unit coverage + an example under docs/examples/

References

• SpanKind spec
• Messaging spans semconv
• pkg/synth/engine.go:449-455 — kind selection
• docs/examples/span-links.yaml — existing producer/consumer linking example