This change deprecates the `target_groups` option in favor
of `static_configs`. The old configuration is still accepted
but prints a warning.
Configuration loading errors if both options are set.
With the blackbox exporter, the instance label will commonly
be used for things other than hostnames so remove this restriction.
https://example.com or https://example.com/probe/me are some examples.
To prevent user error, check that urls aren't provided as targets
when there's no relabelling that could potentically fix them.