prometheus-frr-exporter: init prometheus exporter module

(cherry picked from commit 578c5b6)

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

@@ -40,6 +40,7 @@ let
     "flow"
     "fritz"
     "fritzbox"
+    "frr"
     "graphite"
     "idrac"
     "imap-mailstat"

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

@@ -0,0 +1,49 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+let
+  cfg = config.services.prometheus.exporters.frr;
+  inherit (lib)
+    mkOption
+    types
+    concatStringsSep
+    concatMapStringsSep
+    ;
+in
+{
+  port = 9342;
+  extraOpts = {
+    enabledCollectors = mkOption {
+      type = types.listOf types.str;
+      default = [ ];
+      example = [ "vrrp" ];
+      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 = [ "bfd" ];
+      description = ''
+        Collectors to disable which are enabled by default.
+      '';
+    };
+  };
+  serviceOpts = {
+    serviceConfig = {
+      DynamicUser = false;
+      RuntimeDirectory = "prometheus-frr-exporter";
+      ExecStart = ''
+        ${lib.getExe pkgs.prometheus-frr-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}
+      '';
+    };
+  };
+}