Add meteocat exporter

Allows us to track ambient temperature changes and estimate the
temperature delta between the server room and exterior temperature.
We should be able to predict when we would need to stop the machines due
to excesive temperature as summer approaches.

Reviewed-by: Aleix Boné <abonerib@bsc.es>

pkgs/overlay.nix

@@ -54,4 +54,5 @@ final: prev:
   });
 
   prometheus-slurm-exporter = prev.callPackage ./slurm-exporter.nix { };
+  meteocat-exporter = prev.callPackage ./meteocat-exporter/default.nix { };
 }