nixos/prometheus-nut-exporter: use extraFlags, add nutVariables option

nixos/modules/services/monitoring/prometheus/exporters/nut.nix

@@ -36,6 +36,17 @@ in
         provisioned outside of Nix store.
       '';
     };
+    nutVariables = mkOption {
+      type = types.listOf types.str;
+      default = [ ];
+      description = ''
+        List of NUT variable names to monitor.
+
+        If no variables are set, all numeric variables will be exported automatically.
+        See the [upstream docs](https://github.com/DRuggeri/nut_exporter?tab=readme-ov-file#variables-and-information)
+        for more information.
+      '';
+    };
   };
   serviceOpts = {
     script = ''
@@ -44,7 +55,9 @@ in
       ${pkgs.prometheus-nut-exporter}/bin/nut_exporter \
         --nut.server=${cfg.nutServer} \
         --web.listen-address="${cfg.listenAddress}:${toString cfg.port}" \
-        ${optionalString (cfg.nutUser != "") "--nut.username=${cfg.nutUser}"}
+        ${optionalString (cfg.nutUser != "") "--nut.username=${cfg.nutUser}"} \
+        ${optionalString (cfg.nutVariables != []) "--nut.vars_enable=${concatStringsSep "," cfg.nutVariables}"} \
+        ${concatStringsSep " " cfg.extraFlags}
     '';
   };
 }