As the configuration for the exporters and alertmanager is unchanged
between the two major versions this patch tries to minimize
duplication while at the same time as there's no upgrade path from 1.x
to 2.x, it allows running the two services in parallel. See also #56037
With `promtool` we can check the validity of a configuration before
deploying it. This avoids situations where you would end up with a
broken monitoring system without noticing it - since the monitoring
broke down. :-)
...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).
