...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.
The structured options are incomplete compared to upstream and I think
it will be a maintenance burden to try to keep up. Instead, provide an
option for the raw config file contents (prometheus.yml).
