nixos/prometheus-exporters/knot: update for new exporter

The new exporter has proper console scripts definition, that sets up
another executable name.

The package now also shells out to pidof, which is why we require procps
in the unit PATH.

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

@@ -8,9 +8,9 @@ in {
   port = 9433;
   extraOpts = {
     knotLibraryPath = mkOption {
-      type = types.str;
-      default = "${pkgs.knot-dns.out}/lib/libknot.so";
-      defaultText = literalExpression ''"''${pkgs.knot-dns.out}/lib/libknot.so"'';
+      type = types.nullOr types.str;
+      default = null;
+      example = literalExpression ''"''${pkgs.knot-dns.out}/lib/libknot.so"'';
       description = lib.mdDoc ''
         Path to the library of `knot-dns`.
       '';
@@ -25,7 +25,7 @@ in {
     };
 
     knotSocketTimeout = mkOption {
-      type = types.int;
+      type = types.ints.positive;
       default = 2000;
       description = lib.mdDoc ''
         Timeout in seconds.
@@ -33,17 +33,22 @@ in {
     };
   };
   serviceOpts = {
+    path = with pkgs; [
+      procps
+    ];
     serviceConfig = {
       ExecStart = ''
-        ${pkgs.prometheus-knot-exporter}/bin/knot_exporter \
+        ${pkgs.prometheus-knot-exporter}/bin/knot-exporter \
           --web-listen-addr ${cfg.listenAddress} \
           --web-listen-port ${toString cfg.port} \
-          --knot-library-path ${cfg.knotLibraryPath} \
           --knot-socket-path ${cfg.knotSocketPath} \
           --knot-socket-timeout ${toString cfg.knotSocketTimeout} \
+          ${lib.optionalString (cfg.knotLibraryPath != null) "--knot-library-path ${cfg.knotLibraryPath}"} \
           ${concatStringsSep " \\\n  " cfg.extraFlags}
       '';
-      SupplementaryGroups = [ "knot" ];
+      SupplementaryGroups = [
+        "knot"
+      ];
       RestrictAddressFamilies = [
         # Need AF_UNIX to collect data
         "AF_UNIX"