Add content negotiation support and sharding parameters for Query Frontend

[annanay25]

What this PR does:
This PR is the first half the work to introduce a Query Frontend (follow up of #362), and does a bunch of things:

• Add parameters to HTTP endpoint of the Querier, which will help with sharding. List of parameters -
  • BlockStart & BlockEnd - to specify the shard space for each querier
  • QueryIngesters - to specify whether a querier should query ingesters (only one of the queriers should query ingesters for every query)
• Add content negotiation between Tempo-query and Tempo-querier to marshal trace data into proto (and save a roundtrip b/n proto & json)

Which issue(s) this PR fixes:
Fixes #NA

Checklist

• Tests updated
• NA Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[joe-elliott]

So there's like 3 things going on here

1. Additional parameters on the query endpoint
2. Content negotiation to proto for the HTTP endpoint
3. GRPC for the querier

Given that we know the query frontend is going to want to use HTTP I think it makes sense to drop 3 and only do 1 and 2. Thoughts?

[annanay25]

Given that we know the query frontend is going to want to use HTTP I think it makes sense to drop 3 and only do 1 and 2. Thoughts?

Hmm, Frontend is going to roundtrip GRPC to the Queriers no? It's going to reuse this - https://github.com/cortexproject/cortex/blob/master/pkg/frontend/v2/frontendv2pb/frontend.proto

Content negotiation to proto for the HTTP endpoint

This should be dropped. Was added in the PR by mistake. I don't think we need that anymore.

[joe-elliott]

I don't think the query-frontend supports GRPC. Isn't it all built around that http wrapped in grpc trick? Don't we need http on the queriers to use it without major changes?

[annanay25]

We can add a GRPC handler to the query frontend. QF <> Querier will be GRPC bi-directional stream. Just the payload is HTTP, but we can marshal proto into the body like we're doing between QF and Jaeger-GRPC plugin.

[joe-elliott]

Some clean up requested. This is definitely on the right path!

[joe-elliott]

A few small changes. Looking very good.

[annanay25]

Addressed comments. @joe-elliott - This should be ready for review.