This will store the supposedly secret URL in the Nix Store and
print a honking big warning about it.

Only dashboards in this dir

This commit does not change the semantics, only the looks.

At least it's easy to turn on

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.

Fewer lines almost always better.  Also, we don't want to spread
these out unless we have to.

Seems this is opt-out:
https://github.com/NixOS/nixpkgs/blob/9d1350d9d56411b685ff3de5839ed6728b1bf808/nixos/modules/services/monitoring/grafana.nix#L576-L582