Allow new users to log in using Google OAuth2 authentication

nixos/modules/monitoring/server/grafana.nix

@@ -11,6 +11,9 @@ let
                   anonymous.enable = true;
                 } else {
                   google.enable = true;
+                  # Grafana considers it "sign up" to let in a user it has
+                  # never seen before.
+                  google.allowSignUp = true;
                   google.clientSecretFile = cfg.googleOAuthClientSecretFile;
                   google.clientId = cfg.googleOAuthClientID;
                 };