gRPC ClientCalls and thus gax currently wait for trailers to resolve unary call futures. I believe the original reason for this was to mitigate misconfigured servers where a server endpoint was changed to be server streaming, but the client still expects a unary method. We measured the cost of this safety net to be O(hundreds of millis). For low latency services like Bigtable, this is very high.

This PR is incomplete, but is meant to be a conversation starter. I would like to get gax's opinion on this and guidance how to proceed. Some initial proposals:
1. productionize this PR and roll it out
2. gate this behavior using a flag in UnaryCallSettings
3. expose a bit more surface in gax to allow cloud bigtable to build our callable chains (the current blocker is that GrpcUnaryRequestParamCallable & GrpcExceptionCallable are package private

…railers