Add custom nix-daemon exporter

Allows us to see which derivations are being built in realtime. It is a
bit of a hack, but it seems to work. We simply look at the environment
of the child processes of nix-daemon (usually bash) and then look for
the $name variable which should hold the current derivation being
built. Needs root to be able to read the environ file of the different
nix-daemon processes as they are owned by the nixbld* users.

See: https://discourse.nixos.org/t/query-ongoing-builds/23486
Reviewed-by: Aleix Boné <abonerib@bsc.es>

m/hut/nix-daemon-exporter.nix

@@ -0,0 +1,23 @@
+{ pkgs, config, lib, ... }:
+let
+  script = pkgs.runCommand "nix-daemon-exporter.sh" { }
+    ''
+      cp ${./nix-daemon-builds.sh} $out;
+      chmod +x $out
+    ''
+  ;
+in
+{
+  systemd.services.nix-daemon-exporter = {
+    description = "Daemon to export nix-daemon metrics";
+    path = [ pkgs.procps pkgs.ripgrep ];
+    wantedBy = [ "default.target" ];
+    serviceConfig = {
+      Type = "simple";
+      ExecStart = "${pkgs.socat}/bin/socat TCP4-LISTEN:9999,fork EXEC:${script}";
+      # Needed root to read the environment, potentially unsafe
+      User = "root";
+      Group = "root";
+    };
+  };
+}