Selectively disable Indexing of labels

Prometheus is one of the standard-bearing open-source solutions for monitoring and observability. As a Prometheus-as-a-Service platform, Cortex fills in all of these crucial gaps with aplomb and thus provides a complete out-of-the-box solution for even the most demanding monitoring and observability use cases. Currently all querying labels are indexed. This is IO operation and storage heavy. The idea is to reduce that.