feat: add basic prometheus metrics integration

internal/proxy/director/layer/queue/metrics.go

@@ -0,0 +1,31 @@
+package queue
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+const (
+	metricNamespace  = "bouncer_layer_queue"
+	metricLabelProxy = "proxy"
+	metricLabelLayer = "layer"
+)
+
+var (
+	metricQueueSessions = promauto.NewGaugeVec(
+		prometheus.GaugeOpts{
+			Name:      "sessions",
+			Help:      "Bouncer's queue layer current sessions",
+			Namespace: metricNamespace,
+		},
+		[]string{metricLabelProxy, metricLabelLayer},
+	)
+	metricQueueCapacity = promauto.NewGaugeVec(
+		prometheus.GaugeOpts{
+			Name:      "capacity",
+			Help:      "Bouncer's queue layer capacity",
+			Namespace: metricNamespace,
+		},
+		[]string{metricLabelProxy, metricLabelLayer},
+	)
+)