format && add node monitoring

modules/common/default.nix

@@ -1,6 +1,7 @@
 { pkgs, ... }:
 {
   imports = [
+    ./monitoring.nix
     ./users.nix
   ];
 

modules/common/monitoring.nix

@@ -0,0 +1,9 @@
+{ ... }:
+{
+  services.prometheus.exporters = {
+    node = {
+      enable = true;
+      enabledCollectors = [ "systemd" ];
+    };
+  };
+}