Add EventBroadcaster metrics for observability and debugging

Implement comprehensive metrics tracking for EventBroadcaster implementations:

- 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

Closes #22

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

nats_eventbus.go

@@ -66,6 +66,8 @@ func (neb *NATSEventBus) Subscribe(namespaceID 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(namespaceID)
 		} else {
 			neb.subscriptions = append(neb.subscriptions, sub)
 			log.Printf("[NATSEventBus] Node %s subscribed to %s", neb.nodeID, subject)
@@ -109,13 +111,16 @@ func (neb *NATSEventBus) handleNATSEvent(msg *nats.Msg) {
 		return
 	}
 
+	// Record receive from NATS (cross-node event)
+	neb.metrics.RecordReceive(eventMsg.NamespaceID)
+
 	// Forward to local EventBus subscribers
 	neb.EventBus.Publish(eventMsg.NamespaceID, eventMsg.Event)
 }
 
 // Publish publishes an event both locally and to NATS for cross-node broadcasting
 func (neb *NATSEventBus) Publish(namespaceID string, event *Event) {
-	// First publish locally
+	// First publish locally (this also records metrics)
 	neb.EventBus.Publish(namespaceID, event)
 
 	// Then publish to NATS for other nodes
@@ -130,11 +135,13 @@ 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
 	}
 }