Add loopback check

outbound/lookback.go

@@ -0,0 +1,14 @@
+package outbound
+
+import "context"
+
+type outboundTagKey struct{}
+
+func ContextWithTag(ctx context.Context, outboundTag string) context.Context {
+	return context.WithValue(ctx, outboundTagKey{}, outboundTag)
+}
+
+func TagFromContext(ctx context.Context) (string, bool) {
+	value, loaded := ctx.Value(outboundTagKey{}).(string)
+	return value, loaded
+}