sandbox: add stackdriver scope

This changes the sandbox VMs to allow them to write to StackDriver
metrics.

Updates golang/go#25224
Updates golang/go#38530

Change-Id: I82954b8eed3664289f5c69c0f5301a72206f0948
Reviewed-on: https://go-review.googlesource.com/c/playground/+/229681
Run-TryBot: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/sandbox/sandbox.tf b/sandbox/sandbox.tf
index 49bf8d5..4b8f839 100644
--- a/sandbox/sandbox.tf
+++ b/sandbox/sandbox.tf
@@ -45,7 +45,7 @@
     network = "golang"
   }
   service_account {
-    scopes = ["logging-write", "storage-ro"]
+    scopes = ["logging-write", "storage-ro", "monitoring-write"]
   }
   disk {
     source_image = data.google_compute_image.cos.self_link