nixos/prometheus-blackbox-exporter: add config check

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

@@ -4,6 +4,13 @@ with lib;
 
 let
   cfg = config.services.prometheus.exporters.blackbox;
+
+  checkConfig = file: pkgs.runCommand "checked-blackbox-exporter.conf" {
+    preferLocalBuild = true;
+    buildInputs = [ pkgs.buildPackages.prometheus-blackbox-exporter ]; } ''
+    ln -s ${file} $out
+    blackbox_exporter --config.check --config.file $out
+  '';
 in
 {
   port = 9115;
@@ -21,7 +28,7 @@ in
       ExecStart = ''
         ${pkgs.prometheus-blackbox-exporter}/bin/blackbox_exporter \
           --web.listen-address ${cfg.listenAddress}:${toString cfg.port} \
-          --config.file ${cfg.configFile} \
+          --config.file ${checkConfig cfg.configFile} \
           ${concatStringsSep " \\\n  " cfg.extraFlags}
       '';
       ExecReload = "${pkgs.coreutils}/bin/kill -HUP $MAINPID";