There is the possibility of grpc, but there hasn't been a case for it yet (hence why none of our docs even mention it). Google also loves talking grpc/stubby internally, so any Google internal application to application calls would use that.
Not a Googler, this is what I understood from reading similar things over multiple posts. Feel free to correct me.
https://developers.googleblog.com/2015/02/introducing-grpc-n...