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>

eventbus.go

@@ -24,6 +24,13 @@ type EventBroadcaster interface {
 	SubscriberCount(namespaceID string) int
 }
 
+// MetricsProvider is an optional interface that EventBroadcaster implementations
+// can implement to expose metrics.
+type MetricsProvider interface {
+	// Metrics returns the metrics collector for this broadcaster.
+	Metrics() BroadcasterMetrics
+}
+
 // subscription represents a single subscriber channel with its pattern
 type subscription struct {
 	pattern string
@@ -45,6 +52,7 @@ type EventBus struct {
 	mutex               sync.RWMutex
 	ctx                 context.Context
 	cancel              context.CancelFunc
+	metrics             *DefaultMetricsCollector
 }
 
 // NewEventBus creates a new event bus
@@ -55,9 +63,15 @@ func NewEventBus() *EventBus {
 		wildcardSubscribers: make([]subscription, 0),
 		ctx:                 ctx,
 		cancel:              cancel,
+		metrics:             NewMetricsCollector(),
 	}
 }
 
+// Metrics returns the metrics collector for this event bus.
+func (eb *EventBus) Metrics() BroadcasterMetrics {
+	return eb.metrics
+}
+
 // Subscribe creates a new subscription channel for a namespace pattern.
 // Patterns follow NATS subject matching conventions:
 //   - "*" matches a single token (any sequence without ".")
@@ -84,6 +98,9 @@ func (eb *EventBus) Subscribe(namespacePattern string) <-chan *Event {
 		eb.exactSubscribers[namespacePattern] = append(eb.exactSubscribers[namespacePattern], ch)
 	}
 
+	// Record subscription metric
+	eb.metrics.RecordSubscribe(namespacePattern)
+
 	return ch
 }
 
@@ -98,6 +115,8 @@ func (eb *EventBus) Unsubscribe(namespacePattern string, ch <-chan *Event) {
 			if sub.ch == ch {
 				eb.wildcardSubscribers = append(eb.wildcardSubscribers[:i], eb.wildcardSubscribers[i+1:]...)
 				close(sub.ch)
+				// Record unsubscription metric
+				eb.metrics.RecordUnsubscribe(namespacePattern)
 				break
 			}
 		}
@@ -109,6 +128,8 @@ func (eb *EventBus) Unsubscribe(namespacePattern string, ch <-chan *Event) {
 				// Remove channel from slice
 				eb.exactSubscribers[namespacePattern] = append(subs[:i], subs[i+1:]...)
 				close(subscriber)
+				// Record unsubscription metric
+				eb.metrics.RecordUnsubscribe(namespacePattern)
 				break
 			}
 		}
@@ -128,14 +149,19 @@ func (eb *EventBus) Publish(namespaceID string, event *Event) {
 	eb.mutex.RLock()
 	defer eb.mutex.RUnlock()
 
+	// Record publish metric
+	eb.metrics.RecordPublish(namespaceID)
+
 	// Deliver to exact subscribers
 	subscribers := eb.exactSubscribers[namespaceID]
 	for _, ch := range subscribers {
 		select {
 		case ch <- event:
 			// Event delivered
+			eb.metrics.RecordReceive(namespaceID)
 		default:
 			// Channel full, skip this subscriber (non-blocking)
+			eb.metrics.RecordDroppedEvent(namespaceID)
 		}
 	}
 
@@ -145,8 +171,10 @@ func (eb *EventBus) Publish(namespaceID string, event *Event) {
 			select {
 			case sub.ch <- event:
 				// Event delivered
+				eb.metrics.RecordReceive(namespaceID)
 			default:
 				// Channel full, skip this subscriber (non-blocking)
+				eb.metrics.RecordDroppedEvent(namespaceID)
 			}
 		}
 	}
@@ -159,16 +187,18 @@ func (eb *EventBus) Stop() {
 
 	eb.cancel()
 
-	// Close all exact subscriber channels
-	for _, subs := range eb.exactSubscribers {
+	// Close all exact subscriber channels and update metrics
+	for namespaceID, subs := range eb.exactSubscribers {
 		for _, ch := range subs {
 			close(ch)
+			eb.metrics.RecordUnsubscribe(namespaceID)
 		}
 	}
 
-	// Close all wildcard subscriber channels
+	// Close all wildcard subscriber channels and update metrics
 	for _, sub := range eb.wildcardSubscribers {
 		close(sub.ch)
+		eb.metrics.RecordUnsubscribe(sub.pattern)
 	}
 
 	eb.exactSubscribers = make(map[string][]chan *Event)