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>

go.mod

@@ -1,16 +1,26 @@
 module git.flowmade.one/flowmade-one/aether
 
-go 1.23
+go 1.23.0
 
 require (
 	github.com/google/uuid v1.6.0
 	github.com/nats-io/nats.go v1.37.0
+	github.com/prometheus/client_golang v1.23.2
 )
 
 require (
-	github.com/klauspost/compress v1.17.2 // indirect
+	github.com/beorn7/perks v1.0.1 // indirect
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
+	github.com/klauspost/compress v1.18.0 // indirect
+	github.com/kr/text v0.2.0 // indirect
+	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
 	github.com/nats-io/nkeys v0.4.7 // indirect
 	github.com/nats-io/nuid v1.0.1 // indirect
+	github.com/prometheus/client_model v0.6.2 // indirect
+	github.com/prometheus/common v0.66.1 // indirect
+	github.com/prometheus/procfs v0.16.1 // indirect
+	go.yaml.in/yaml/v2 v2.4.2 // indirect
 	golang.org/x/crypto v0.18.0 // indirect
-	golang.org/x/sys v0.16.0 // indirect
+	golang.org/x/sys v0.35.0 // indirect
+	google.golang.org/protobuf v1.36.8 // indirect
 )