nixos/prometheus-exporters: rewrite and restructure

- prometheus exporters are now configured with
  `services.prometheus.exporters.<name>`
- the exporters are now defined by attribute sets
  from which the options for each exporter are generated
- most of the exporter definitions are used unchanged,
  except for some changes that should't have any impact
  on the functionality.

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

@@ -0,0 +1,39 @@
+{ config, lib, pkgs }:
+
+with lib;
+
+let
+  cfg = config.services.prometheus.exporters.node;
+in
+{
+  port = 9100;
+  extraOpts = {
+    enabledCollectors = mkOption {
+      type = types.listOf types.string;
+      default = [];
+      example = ''[ "systemd" ]'';
+      description = ''
+        Collectors to enable. The collectors listed here are enabled in addition to the default ones.
+      '';
+    };
+    disabledCollectors = mkOption {
+      type = types.listOf types.str;
+      default = [];
+      example = ''[ "timex" ]'';
+      description = ''
+        Collectors to disable which are enabled by default.
+      '';
+    };
+  };
+  serviceOpts = {
+    serviceConfig = {
+      ExecStart = ''
+        ${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 " \\\n  " cfg.extraFlags}
+      '';
+    };
+  };
+}