add conn limiter

limiter/traffic/limiter.go

@@ -0,0 +1,15 @@
+package traffic
+
+import "context"
+
+type Limiter interface {
+	// Wait blocks with the requested n and returns the result value,
+	// the returned value is less or equal to n.
+	Wait(ctx context.Context, n int) int
+	Limit() int
+}
+
+type TrafficLimiter interface {
+	In(key string) Limiter
+	Out(key string) Limiter
+}