prometheus module: add blackboxExporter

nixos/modules/module-list.nix

···
   ./services/monitoring/nagios.nix
   ./services/monitoring/prometheus/default.nix
   ./services/monitoring/prometheus/alertmanager.nix
+  ./services/monitoring/prometheus/blackbox-exporter.nix
   ./services/monitoring/prometheus/json-exporter.nix
   ./services/monitoring/prometheus/nginx-exporter.nix
   ./services/monitoring/prometheus/node-exporter.nix

nixos/modules/services/monitoring/prometheus/blackbox-exporter.nix

···
+{ config, pkgs, lib, ... }:
+
+with lib;
+
+let
+  cfg = config.services.prometheus.blackboxExporter;
+in {
+  options = {
+    services.prometheus.blackboxExporter = {
+      enable = mkEnableOption "prometheus blackbox exporter";
+
+      configFile = mkOption {
+        type = types.path;
+        description = ''
+          Path to configuration file.
+        '';
+      };
+
+      port = mkOption {
+        type = types.int;
+        default = 9115;
+        description = ''
+          Port to listen on.
+        '';
+      };
+
+      extraFlags = mkOption {
+        type = types.listOf types.str;
+        default = [];
+        description = ''
+          Extra commandline options when launching the blackbox exporter.
+        '';
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    systemd.services.prometheus-blackbox-exporter = {
+      description = "Prometheus exporter for blackbox probes";
+      unitConfig.Documentation = "https://github.com/prometheus/blackbox_exporter";
+      wantedBy = [ "multi-user.target" ];
+      serviceConfig = {
+        User = "nobody";
+        Restart = "always";
+        PrivateTmp = true;
+        WorkingDirectory = /tmp;
+        ExecStart = ''
+          ${pkgs.prometheus-blackbox-exporter}/bin/blackbox_exporter \
+            -web.listen-address :${toString cfg.port} \
+            -config.file ${cfg.configFile} \
+            ${concatStringsSep " \\\n  " cfg.extraFlags}
+        '';
+        ExecReload = "${pkgs.coreutils}/bin/kill -HUP $MAINPID";
+      };
+    };
+  };
+}