Add basic metric sink

internal/app/app.go

@@ -290,6 +290,15 @@ func (a *app) WithEventStream() App {
 			return
 		}
 
+		var metricSink audit.Sink
+		if metricSink, err = sink.NewMetricSink(); err != nil {
+			return
+		}
+
+		if err = eventStream.RegisterSink(a.ctx, metricSink); err != nil {
+			return
+		}
+
 		a.ctx = context.WithValue(a.ctx, eventStreamKey, eventStream)
 		return
 	})

pkg/audit/sink/metric_sink.go

@@ -0,0 +1,34 @@
+package sink
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"gitlab.com/inetmock/inetmock/pkg/audit"
+	"gitlab.com/inetmock/inetmock/pkg/metrics"
+)
+
+func NewMetricSink() (sink audit.Sink, err error) {
+	var totalEventsCounter *prometheus.CounterVec
+	if totalEventsCounter, err = metrics.Counter("audit", "events_total", "", "application", "transport"); err != nil {
+		return
+	}
+	sink = &metricSink{
+		eventCounter: totalEventsCounter,
+	}
+	return
+}
+
+type metricSink struct {
+	eventCounter *prometheus.CounterVec
+}
+
+func (metricSink) Name() string {
+	return "metrics"
+}
+
+func (m metricSink) OnSubscribe(evs <-chan audit.Event) {
+	go func(evs <-chan audit.Event) {
+		for ev := range evs {
+			m.eventCounter.WithLabelValues(ev.Application.String(), ev.Transport.String()).Inc()
+		}
+	}(evs)
+}