feat: include producer metadata in per-message events

[yordis]

• Per-message telemetry events were missing the `producer` field that batch-level events already had (added in Add the producer to the telemetry metadata #308), making it impossible to correlate per-message spans back to their source producer
• Without producer metadata, consumers of per-message telemetry events cannot populate OTel semantic convention attributes like `messaging.system` (derived from the producer module) and `messaging.destination.name` (from producer opts), which are required for meaningful span scoping in multi-producer pipelines

[josevalim]

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