b20fdff521
...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 *** NixOS is a Linux distribution based on the purely functional package management system Nix. More information can be found at http://nixos.org/nixos and in the manual in doc/manual.