feat(eventdrop): add centralized telemetry API for dropped events

[jijo-OO7]

WIP: Phase 1 – Centralized telemetry API for dropped events (#8572)

Description

This PR establishes the foundation for dropped event telemetry in the Broker data plane by introducing a centralized RecordEventDropped API. The package provides OpenTelemetry instrumentation (metrics and traces) for drop sites without modifying existing handler code.

This is Phase 1 - API definition only. Handler integration happens in a follow-up PR once the design is validated.

Changes

• New pkg/eventdrop package with:
  • Reason enum capturing drop causes (TTLMissing, TTLExhausted)
  • Info struct for drop context metadata
  • RecordEventDropped() entry point for unified telemetry
  • OTel metrics (low-cardinality) and traces (high-cardinality)
  • Comprehensive test coverage

Telemetry Strategy

Following Evan's guidance in #8572:

Metrics: Low-cardinality labels only

• eventing_broker_events_dropped_total counter
• Labels: namespace, broker, trigger, reason
• EventType/EventSource excluded to avoid cardinality explosion

Traces: Full context via sampling

• event-dropped event on active spans
• Attributes: all metadata including eventType, eventSource, details
• Safe due to trace sampling

Design Notes

• Graceful degradation: OTEL init failures don't disrupt broker operation
• Flexible Info struct: Not all fields required at all call sites (e.g., Trigger empty at ingress)
• Single entry point: Ensures consistency and simplifies future extensions
• No behavior changes: Telemetry only, existing flow unchanged

Integration (Phase 1.5)

Follow-up PR will wire this into:

• pkg/broker/filter/filter_handler.go (ReasonTTLMissing)
• pkg/broker/ingress/ingress_handler.go (ReasonTTLExhausted) Separate PR allows design review before touching handlers.

Testing

Phase 1 validates API surface and graceful degradation. Deep OTel verification deferred until Phase 1.5 integration.

cc/ @evankanderson (per #8572 guidance) @Cali0707 @matzew @creydr - Design feedback before proceeding to handler wiring would be valuable.

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