Add readiness handlers to distributor and querier modules

[sboschman]

Is your feature request related to a problem? Please describe.
Currently only the ingester has a readiness handler (https://github.com/grafana/loki/blob/master/pkg/loki/modules.go#L148). If you deploy the three Loki components (distributor, ingester and querier) separately not all of them have a readiness handler to use as Kubernetes readiness/liveness probe paths. For the querier the path /api/prom/label could be used as readiness/liveness path. The readiness/liveness probes for the distributor had to be disabled though.

Describe the solution you'd like
Separate ready handlers for all three modules.

[jan25]

@Kuqd would you know if someone is already working on this? Otherwise i'm happy to take a look

[cyriltovena]

Nobody is feel free to take it.

[adityacs]

@cyriltovena I would like to take this.

[jan25]

@adityacs I couldn't work on this. Go ahead!

[cyriltovena]

Please do ! I pinned the issue because we need this rather sooner than later.