TUN-8701: Add metrics and adjust logs for datagram v3

Closes TUN-8701

quic/v3/metrics.go

@@ -0,0 +1,90 @@
+package v3
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+const (
+	namespace = "cloudflared"
+	subsystem = "udp"
+)
+
+type Metrics interface {
+	IncrementFlows()
+	DecrementFlows()
+	PayloadTooLarge()
+	RetryFlowResponse()
+	MigrateFlow()
+}
+
+type metrics struct {
+	activeUDPFlows     prometheus.Gauge
+	totalUDPFlows      prometheus.Counter
+	payloadTooLarge    prometheus.Counter
+	retryFlowResponses prometheus.Counter
+	migratedFlows      prometheus.Counter
+}
+
+func (m *metrics) IncrementFlows() {
+	m.totalUDPFlows.Inc()
+	m.activeUDPFlows.Inc()
+}
+
+func (m *metrics) DecrementFlows() {
+	m.activeUDPFlows.Dec()
+}
+
+func (m *metrics) PayloadTooLarge() {
+	m.payloadTooLarge.Inc()
+}
+
+func (m *metrics) RetryFlowResponse() {
+	m.retryFlowResponses.Inc()
+}
+
+func (m *metrics) MigrateFlow() {
+	m.migratedFlows.Inc()
+}
+
+func NewMetrics(registerer prometheus.Registerer) Metrics {
+	m := &metrics{
+		activeUDPFlows: prometheus.NewGauge(prometheus.GaugeOpts{
+			Namespace: namespace,
+			Subsystem: subsystem,
+			Name:      "active_flows",
+			Help:      "Concurrent count of UDP flows that are being proxied to any origin",
+		}),
+		totalUDPFlows: prometheus.NewCounter(prometheus.CounterOpts{
+			Namespace: namespace,
+			Subsystem: subsystem,
+			Name:      "total_flows",
+			Help:      "Total count of UDP flows that have been proxied to any origin",
+		}),
+		payloadTooLarge: prometheus.NewCounter(prometheus.CounterOpts{
+			Namespace: namespace,
+			Subsystem: subsystem,
+			Name:      "payload_too_large",
+			Help:      "Total count of UDP flows that have had origin payloads that are too large to proxy",
+		}),
+		retryFlowResponses: prometheus.NewCounter(prometheus.CounterOpts{
+			Namespace: namespace,
+			Subsystem: subsystem,
+			Name:      "retry_flow_responses",
+			Help:      "Total count of UDP flows that have had to send their registration response more than once",
+		}),
+		migratedFlows: prometheus.NewCounter(prometheus.CounterOpts{
+			Namespace: namespace,
+			Subsystem: subsystem,
+			Name:      "migrated_flows",
+			Help:      "Total count of UDP flows have been migrated across local connections",
+		}),
+	}
+	registerer.MustRegister(
+		m.activeUDPFlows,
+		m.totalUDPFlows,
+		m.payloadTooLarge,
+		m.retryFlowResponses,
+		m.migratedFlows,
+	)
+	return m
+}