nixos/prometheus-exporters/kea: run under same user/group as kea

This fixes access to the kea unix sockets, after enabling
RuntimeDirectoryPreserve on the kea units.

https://github.com/NixOS/nixpkgs/pull/274460#issuecomment-1869702893

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

@@ -31,13 +31,15 @@ in {
     ];
     serviceConfig = {
       User = "kea";
+      DynamicUser = true;
       ExecStart = ''
         ${pkgs.prometheus-kea-exporter}/bin/kea-exporter \
           --address ${cfg.listenAddress} \
           --port ${toString cfg.port} \
           ${concatStringsSep " " cfg.controlSocketPaths}
       '';
-      SupplementaryGroups = [ "kea" ];
+      RuntimeDirectory = "kea";
+      RuntimeDirectoryPreserve = true;
       RestrictAddressFamilies = [
         # Need AF_UNIX to collect data
         "AF_UNIX"