feat(kafka): add span links for pub/sub tracing

[VIDHITTS]

Description:

• Implements OpenTelemetry span links for Kafka pub/sub tracing
• Addresses issue Support Span links for PubSub Tracing #2650
• Enables proper async correlation between producers and consumers following OTel messaging semantic conventions. Previously, subscriber spans created new disconnected traces. Now they create spans with links to producer spans.

Breaking Changes (if applicable):

• No breaking changes. Kafka messages now include trace context headers, which are ignored by consumers not expecting them.

Additional Information:

• Uses go.opentelemetry.io/otel for trace context propagation
• W3C TraceContext format for header injection/extraction
• This is the first phase (Kafka only). Other backends will follow in separate PRs.

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!

[coolwednesday]

Please fix the title of the PR.

[VIDHITTS]

@coolwednesday , have renamed the title

[coolwednesday]

@coolwednesday , have renamed the title

Thanks ! Will review.

[coolwednesday]

The tracer name is currently set to "gofr". It would be better to use a constant or a more specific package-level name (for example, gofr.dev/pkg/gofr/datasource/pubsub/kafka). This makes it easier for users to filter and identify traces by the source library.

Verifying span links with GoFr + Jaeger (Docker)

To validate your span link implementation:

• Jaeger stores spans with links and converts those links into references (typically FOLLOWS_FROM).
• You can inspect these in the Jaeger UI at localhost:16686 under span details.
• Jaeger does not show a dedicated “span link” label — links appear as normal references.

If you see additional FOLLOWS_FROM references, it indicates your span links are being exported correctly.

[coolwednesday]

@VIDHITTS, Also, Please do add screenshot.

[VIDHITTS]

Span Links Verification with Jaeger

The screenshot shows the kafka-subscribe span in Jaeger UI with:

• Service: kafka-span-links-demo
• Tags:
  • messaging.system = kafka
  • messaging.operation = receive
  • messaging.destination.name = test-span-links
• References (1): FOLLOWS_FROM link to the producer span (SpanID: 76d60bffb8ea893c)

This confirms that:

1. Producer injects W3C trace context into Kafka headers
2. Consumer extracts trace context and creates a span linked to the producer
3. Span links appear as FOLLOWS_FROM references in Jaeger (as expected per OTel messaging conventions)

[VIDHITTS]

Hi @coolwednesday and @Umang01-hash,

Thank you for reviewing and merging the Kafka implementation!

Since the Kafka phase is finalized, I am ready to generalize this approach for the remaining pub/sub backends (Google Pub/Sub, NATS, SQS, etc.) to fully close out this issue.

How would you prefer I handle the remaining implementations?

Raise one combined PR for all remaining backends?

Raise individual, separate PRs for each backend?
Let me know how you'd like me to proceed with the backends!