Support Span links for PubSub Tracing

[coolwednesday]

Key Highlights:

Why This Matters:

• GoFr currently uses parent-child tracing which doesn't fit asynchronous pub/sub patterns
• OpenTelemetry recommends span links as the standard for messaging systems
• Span links enable proper correlation across decoupled, asynchronous message flows

What It Proposes:

1. Producer-side enhancements: Inject W3C trace context into message metadata
2. Consumer-side enhancements: Extract context and create span links to producer spans
3. Batch operation support: Create appropriate span hierarchies with multiple links

Technical Implementation:

• Uses trace.WithLinks() from OpenTelemetry Go SDK
• Follows messaging semantic conventions
• Supports multiple backends (Kafka, Google Pub/Sub, NATS)
• Maintains backward compatibility with configuration options

Benefits:

• ✅ Correct async correlation between producers and consumers
• ✅ Supports fan-out (1→many) and fan-in (many→1) patterns
• ✅ Aligns with OpenTelemetry standards
• ✅ Better observability for event-driven architectures