Support lables/{label}/values through the remote read API

[tomwilkie]

From IRC:

Main reason I noticed was I was using the metric() param in Grafana which reads from label/%/values. Wasn't sure if it was intentional or not that the end point didn't hit remote read. I could kinda imagine arguments for and against :)

We'll probably want to move to gRPC before we do this, to make the API handling simpler.

[AlekSi]

I think /api/v1/series can be added to this issue too.

[lgogolin]

Any update on this ?

Getting labels from remote_read would be really helpful

[kkotamar]

This feature will be very useful, if prometheus data is read from remote completely.

[lgogolin]

@kkotamar don't know what You mean, You can do remote_read from prometheus

Anyway, there was already a PR adding this feature:
#3620

But was closed, because someone said "it would be nice to have gRPC", ok nice but this is opened for almost a year a nothing happened