Add EventBroadcaster metrics for observability and debugging

- Add BroadcasterMetrics interface for reading metrics per namespace
- Add MetricsCollector interface and DefaultMetricsCollector implementation
- Track events_published and events_received counters per namespace
- Track active_subscriptions gauge per namespace
- Track publish_errors, subscribe_errors, and dropped_events counters
- Add MetricsProvider interface for EventBroadcaster implementations
- Integrate metrics tracking into EventBus and NATSEventBus
- Add optional Prometheus integration via PrometheusMetricsAdapter
- Add comprehensive unit tests for metrics functionality

Closes #22

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

nats_eventbus.go

@@ -79,6 +79,8 @@ func (neb *NATSEventBus) Subscribe(namespacePattern string) <-chan *Event {
 		})
 		if err != nil {
 			log.Printf("[NATSEventBus] Failed to subscribe to NATS subject %s: %v", subject, err)
+			// Record subscription error
+			neb.metrics.RecordSubscribeError(namespacePattern)
 		} else {
 			neb.subscriptions = append(neb.subscriptions, sub)
 			if IsWildcardPattern(namespacePattern) {
@@ -147,9 +149,11 @@ func (neb *NATSEventBus) deliverToWildcardSubscribers(pattern string, event *Eve
 		if sub.pattern == pattern {
 			select {
 			case sub.ch <- event:
-				// Event delivered
+				// Event delivered from NATS
+				neb.metrics.RecordReceive(pattern)
 			default:
 				// Channel full, skip this subscriber (non-blocking)
+				neb.metrics.RecordDroppedEvent(pattern)
 			}
 		}
 	}
@@ -172,11 +176,13 @@ func (neb *NATSEventBus) Publish(namespaceID string, event *Event) {
 	data, err := json.Marshal(eventMsg)
 	if err != nil {
 		log.Printf("[NATSEventBus] Failed to marshal event for NATS: %v", err)
+		neb.metrics.RecordPublishError(namespaceID)
 		return
 	}
 
 	if err := neb.nc.Publish(subject, data); err != nil {
 		log.Printf("[NATSEventBus] Failed to publish event to NATS: %v", err)
+		neb.metrics.RecordPublishError(namespaceID)
 		return
 	}
 }