TUN-8861: Rename Session Limiter to Flow Limiter

## Summary
Session is the concept used for UDP flows. Therefore, to make
the session limiter ambiguous for both TCP and UDP, this commit
renames it to flow limiter.

Closes TUN-8861

flow/metrics.go

@@ -0,0 +1,23 @@
+package flow
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+const (
+	namespace = "flow"
+)
+
+var (
+	labels = []string{"flow_type"}
+
+	flowRegistrationsDropped = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: namespace,
+		Subsystem: "client",
+		Name:      "registrations_rate_limited_total",
+		Help:      "Count registrations dropped due to high number of concurrent flows being handled",
+	},
+		labels,
+	)
+)