Add a default check to ensure label values are strings

[douglascamata]

Prometheus won't accept labels with values that aren't strings. This issue is often accidentally hit because YAML parsers try to parse certain unquoted identifiers to built-in types (booleans, integers, etc). So things like true become a boolean and 1 an integer.

[douglascamata]

Hey @prymitive, any chance you could have a look at this, please? 🙏

[prymitive]

Thanks for the PR but this doesn't really belong in a check.
This is part of a parser validation and so I've raised a PR for checks there - #890