Looks interesting, but TBH I'm still waiting for full swagger spec generation. It should be easy to go from grpc->swagger to generate REST client libs. Unfortunately in practice the swagger generator has many holes in it making it useless in several use cases. So... without being able to generate swagger specs it looks like gRPC is no use for a large number of use cases. Shame really since it has promise but still seems half-baked.
That's pretty much exactly what tools like https://github.com/grpc-ecosystem/grpc-gateway are doing