nixos/prometheus-exporters: fix default firewall filter

Instead of always using the default port of one exporter for its default
firewall filter, the port from the current service configuration is used.

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

@@ -84,7 +84,8 @@ let
     };
     firewallFilter = mkOption {
       type = types.str;
-      default = "-p tcp -m tcp --dport ${toString port}";
+      default = "-p tcp -m tcp --dport ${toString cfg.${name}.port}";
+      defaultText = "-p tcp -m tcp --dport ${toString port}";
       example = literalExample ''
         "-i eth0 -p tcp -m tcp --dport ${toString port}"
       '';