Force a certain public address for the server

This is used in constructing the Google OAuth2 login callback URL and has to
match what's configured in Google - also helps if it is actually the address
of the server.

nixos/modules/monitoring/server/grafana.nix

@@ -70,6 +70,12 @@ in {
 
       # No phoning home
       analytics.reporting.enable = false;
+
+      # Force Grafana to believe it is reachable via https on the default port
+      # number because that's where the nginx that forwards traffic to it is
+      # listening.  Grafana's own server listens on an internal address that
+      # doesn't matter to anyone except our nginx instance.
+      rootUrl = "https://%(domain)s/";
     };
 
     services.grafana.auth = {