prometheus service: fix basic auth option

If some configuration is provided, we need to filter out the `_module` key or
else it breaks prometheus.

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

@@ -134,6 +134,7 @@ let
           };
         });
         default = null;
+        apply = x: if x == null then null else _filter x;
         description = ''
           Optional http login credentials for metrics scraping.
         '';