Add prometheus node exporter config

configuration/common/default.nix

@@ -1,6 +1,7 @@
 { config, pkgs, ... }:
 {
   imports = [
+    ./prometheus-node-exporter.nix
     ../../users/colmena-deploy
     ../../users/yuri
   ];

configuration/common/prometheus-node-exporter.nix

@@ -0,0 +1,7 @@
+{ ... }:
+{
+  services.prometheus.exporters.node = {
+    enable = true;
+    openFirewall = true;
+  };
+}