ReplaceAllString only replaces the matching part of the regex,
the unmatched bits around it are left in place. This is not the
expected or desired behaviour as the replacement string should
be everything.
This may break users dependant on this behaviour, but
what they're doing is still possible.
The intended use case is where a user has tags/labels coming
from metadata in Kubernetes or EC2, and wants to make
some subset of them into target labels.
This takes the modulus of a hash of some labels.
Combined with a keep relabel action, this allows
for sharding of targets across multiple prometheus
servers.
This commit adds a relabelling stage on the set of base
labels from which a target is created. It allows to drop
targets and rewrite any regular or internal label.
