We have to explicitely add the port number so the relabel
config can strip it out again.  Else, `instance' seems to
get its value from somewhere else - somewhere our relabel
config does not look at.

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

Known issues:

  - Error rate tooltip computes over whole range (sum), should be per second or per minute or so
  - Duration buckets not great. Should also be per second probably.

We have enough numbers with little value already

Make everything more consistent and generally more nice.
I am a bit proud of this now.
Let's hope it holds up.

Rationale: These are servers, they should be transmitting mostly.
But really it's rather arbitrary.

- Add node exporter to nodes
- Make the old nixpkgs 19.09 node exporter work (amend config)
- Fix a typo (??) and exclude monitoringvpn in network traffic graphs to prevent double counting