Increase grpc limits

[gouthamve]

Seeing this on distributor logs:

level=warn ts=2018-12-06T15:46:43.673737673Z caller=gokit.go:43 msg="POST /api/prom/push (500) 73.943803ms Response: \"rpc error: code = Unknown desc = out of order sample\\n\" ws: false; Accept-Encoding: gzip; Authorization: Basic ***; Connection: close; Content-Length: 140456; Content-Type: application/x-protobuf; User-Agent: Go-http-client/2.0; Via: 1.1 google; X-Cloud-Trace-Context: 7b9223a0b9071768b79f1a3de078970a/2632702499859856938; X-Forwarded-For: 35.199.19.169, 35.190.19.129, 10.15.0.35; X-Forwarded-Proto: https; X-Scope-Orgid: 29; "
level=warn ts=2018-12-06T15:46:55.01588331Z caller=gokit.go:43 msg="POST /api/prom/push (500) 210.404432ms Response: \"rpc error: code = ResourceExhausted desc = grpc: received message larger than max (6842981 vs. 4194304)\\n\" ws: false; Accept-Encoding: gzip; Authorization: Basic ***; Connection: close; Content-Length: 859722; Content-Type: application/x-protobuf; User-Agent: Go-http-client/2.0; Via: 1.1 google; X-Cloud-Trace-Context: 73eb6754952f85d83d676d8181301a9e/8514407636735609549; X-Forwarded-For: 35.194.87.10, 35.190.19.129, 10.15.0.7; X-Forwarded-Proto: https; X-Scope-Orgid: 29; "

[tomwilkie]

I think this was fixed by weaveworks/common#146