TUN-8441: Correct UDP total sessions metric to a counter and add new ICMP metrics

cloudflared_udp_total_sessions was incorrectly a gauge when it
represents the total since the cloudflared process started and will
only ever increase.

Additionally adds new ICMP metrics for requests and replies.

ingress/icmp_metrics.go

@@ -0,0 +1,39 @@
+package ingress
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+const (
+	namespace = "cloudflared"
+)
+
+var (
+	icmpRequests = prometheus.NewCounter(prometheus.CounterOpts{
+		Namespace: namespace,
+		Subsystem: "icmp",
+		Name:      "total_requests",
+		Help:      "Total count of ICMP requests that have been proxied to any origin",
+	})
+	icmpReplies = prometheus.NewCounter(prometheus.CounterOpts{
+		Namespace: namespace,
+		Subsystem: "icmp",
+		Name:      "total_replies",
+		Help:      "Total count of ICMP replies that have been proxied from any origin",
+	})
+)
+
+func init() {
+	prometheus.MustRegister(
+		icmpRequests,
+		icmpReplies,
+	)
+}
+
+func incrementICMPRequest() {
+	icmpRequests.Inc()
+}
+
+func incrementICMPReply() {
+	icmpReplies.Inc()
+}