nixos/prometheus: make scrapeConfigs.*.static_configs.*.labels optional

...by providing a default value of "no labels" (an empty attrset).

Without this change we get

  $ nixos-rebuild test -I nixpkgs=.
  building Nix...
  building the system configuration...
  error: The option `services.prometheus.scrapeConfigs.[definition 1-entry 1].static_configs.[definition 1-entry 1].labels' is used but not defined.

which is unneeded, because labels _are_ optional.

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

@@ -191,6 +191,7 @@ let
       };
       labels = mkOption {
         type = types.attrsOf types.str;
+        default = {};
         description = ''
           Labels assigned to all metrics scraped from the targets.
         '';