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.