TUN-7477: Add UDP/TCP session metrics

New gauge metrics are exposed in the prometheus endpoint to
capture the current and total TCP and UDP sessions that
cloudflared has proxied.

datagramsession/metrics.go

@@ -0,0 +1,40 @@
+package datagramsession
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+const (
+	namespace = "cloudflared"
+)
+
+var (
+	activeUDPSessions = prometheus.NewGauge(prometheus.GaugeOpts{
+		Namespace: namespace,
+		Subsystem: "udp",
+		Name:      "active_sessions",
+		Help:      "Concurrent count of UDP sessions that are being proxied to any origin",
+	})
+	totalUDPSessions = prometheus.NewGauge(prometheus.GaugeOpts{
+		Namespace: namespace,
+		Subsystem: "udp",
+		Name:      "total_sessions",
+		Help:      "Total count of UDP sessions that have been proxied to any origin",
+	})
+)
+
+func init() {
+	prometheus.MustRegister(
+		activeUDPSessions,
+		totalUDPSessions,
+	)
+}
+
+func incrementUDPSessions() {
+	totalUDPSessions.Inc()
+	activeUDPSessions.Inc()
+}
+
+func decrementUDPActiveSessions() {
+	activeUDPSessions.Dec()
+}