playground: warm up backend dialer at start

Reduces latency for new frontend instances.

Updates golang/go#25224

Change-Id: I5847f6d3dbfd09a2f2cc6e71548f55e6bcd31e52
Reviewed-on: https://go-review.googlesource.com/c/playground/+/216737
Reviewed-by: Alexander Rakoczy <alex@golang.org>
diff --git a/main.go b/main.go
index 3bba9ab..6b1105e 100644
--- a/main.go
+++ b/main.go
@@ -54,6 +54,11 @@
 	if port == "" {
 		port = "8080"
 	}
+
+	// Get the backend dialer warmed up. This starts
+	// RegionInstanceGroupDialer queries and health checks.
+	go sandboxBackendClient()
+
 	log.Printf("Listening on :%v ...", port)
 	log.Fatalf("Error listening on :%v: %v", port, http.ListenAndServe(":"+port, s))
 }