nixos/prometheus-node-exporter: use socket activation

Run node_exporter with `--web.systemd-socket` and move the
listen-address/port configuration into a companion `.socket` unit.

Closes https://github.com/NixOS/nixpkgs/issues/510379

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

@@ -39,6 +39,7 @@ in
       '';
     };
   };
+
   serviceOpts = {
     serviceConfig = {
       DynamicUser = false;
@@ -47,7 +48,7 @@ in
         ${pkgs.prometheus-node-exporter}/bin/node_exporter \
           ${concatMapStringsSep " " (x: "--collector." + x) cfg.enabledCollectors} \
           ${concatMapStringsSep " " (x: "--no-collector." + x) cfg.disabledCollectors} \
-          --web.listen-address ${cfg.listenAddress}:${toString cfg.port} ${concatStringsSep " " cfg.extraFlags}
+          --web.systemd-socket ${concatStringsSep " " cfg.extraFlags}
       '';
       RestrictAddressFamilies =
         optionals (collectorIsEnabled "logind" || collectorIsEnabled "systemd") [
@@ -67,4 +68,8 @@ in
       ProtectHome = true;
     };
   };
+
+  socketOpts = {
+    socketConfig.ListenStream = "${cfg.listenAddress}:${toString cfg.port}";
+  };
 }