add OTel span links for MQTT pubsub tracing

[Umang01-hash]

Pull Request Template

Description:

• Adds OpenTelemetry tracing instrumentation to the MQTT pubsub datasource, consistent with the existing implementation for Kafka (feat(kafka): add span links for pub/sub tracing #2952), Google PubSub (feat(google): add span links for pub/sub tracing #3019), and SQS (feat(sqs): add span links for pub/sub tracing #3018).

MQTT 3.1.1 Limitation:

Unlike Kafka, SQS, and Google PubSub — which carry trace context in message headers/attributes — MQTT 3.1.1 has no user properties mechanism. The traceparent injected on publish cannot be transmitted through the broker to subscribers. As a result, subscriber spans are correctly created as orphan consumer spans (no link to the producer span).

Screenshots:

Checklist:

• I have formatted my code using goimport and golangci-lint.
• All new code is covered by unit tests.
• This PR does not decrease the overall code coverage.
• I have reviewed the code comments and documentation for clarity.

Thank you for your contribution!