Add span links in MQTT

[coolwednesday]

• Refer Add span links in SQS #3015 (comment)

[VIDHITTS]

@coolwednesday I would like to work on this.

[majiayu000]

Hi, I've been looking into this and traced the root cause.

Proposed approach: Mirror PR #2952 pattern for MQTT: (1) Add trace context propagation in MQTT publisher (inject W3C traceparent into message metadata), (2) Extract context on subscriber side and create span links to producer spans via trace.WithLinks(), (3) Add unit tests for tracing logic. Files likely: pkg/gofr/datasource/pubsub/mqtt/mqtt.go + new tracing.go + tracing_test.go. Must request assignment first via issue comment per CONTRIBUTING.md policy.

I'll open a draft PR shortly. Happy to adjust based on your preference.

[adityakrmishra]

Hi team! I would like to take this issue on.

I saw that previous PRs (#3187, #3101) were closed because hacking trace propagation into MQTT 3.1.1 via payload wrapping causes silent breaking changes for mixed environments.
I agree with the maintainers' feedback that the correct approach is to utilize MQTT 5.0's User Properties. My proposed approach is:
Upgrade the underlying MQTT driver implementation in Gofr to support MQTT 5.0.
Once upgraded, implement the span links using User Properties to inject and extract the W3C traceparent, mirroring the clean Kafka/SQS implementation without modifying the payload.
Does this sound like the right path forward? If approved, I can start working on this.