Currently, thanos-receive-controller uses the same label to watch receiver pods and update them on hashring changes.

However, in a Thanos receiver router and ingestor setup (https://thanos.io/tip/proposals-accepted/202012-receive-split.md/), while we need to watch the ingestor pods to update the hashring configmap, the hashring configmap gets used in the router pods. In this scenario, the router pods need to be annotated on hashring change instead of the ingestor pods.

For thanos-receive-controller to support this Thanos receiver router+receiver setup, it needs to support an option to specify a separate pod label (different from the pod label used to watch the receiver ingestor pods) to use for annotating pods on hashring change.