Potentially replace parts of Cortex's remote querying code with upstream code once it exists

[jml]

From @tomwilkie

Could probably also be upstreamed.

Copied from original issue: tomwilkie/frankenstein#4

[juliusv]

From email:

@rade:
Is this remote read-back feature what #5 is about? I thought it was but the issue is confusingly (to me) described as "remote query API" when querying cortex remotely is already possible today. So could somebody please add some clarification to that issue, one way or the other.

@juliusv:
I think that issue was more motivated by trying to upstream some of our own remote querying code so that we wouldn't have to maintain it ourselves. But it's strongly related to what would be needed upstream.

I'm going to create another issue for general remote queryability through Prometheus and rename this one.

[tomwilkie]

The prometheus code does a bunch more work with external labels than we need to. Once #713 is merged, we're on all the same interfaces as upstream and I don't see much more work to do. Suggest we close.

[juliusv]

Ok with me.