receive: create a gRPC remote_write endpoint

[blockloop]

I understand the compatibility reasons for creating a remote_write endpoint that adheres to the Prometheus standard. Would you consider enabling a gRPC endpoint to enable pushing metrics from arbitrary gRPC sources? Ideally it would use a gRPC Stream to improve performance.

[brancz]

I think I would want something like this to be experimental, but if we can prove it to have a significant improvement over the existing one, then I'd even go a step further and attempt to make Prometheus send remote write via gRPC.

[blockloop]

I've started work on this here: https://github.com/blockloop/thanos/tree/bjones/receive/use-grpc