This would be a fairly large redesign, but if my initial thoughts are correct, we could possibly get rid of the whole code generator. I may be wrong of course.

Here is the godoc:
https://godoc.org/google.golang.org/grpc#UnaryClientInterceptor

Here is a simple example of using a Client Interceptor to measure time:
https://about.sourcegraph.com/go/grpc-in-production/#client-interceptor
Same for a Server Interceptor:
https://about.sourcegraph.com/go/grpc-in-production/#server-interceptor
Here is a Client Interceptor for retry logic:
https://about.sourcegraph.com/go/grpc-in-production/#networks-fail

Here is a youtube video with more details (not only on inceptors):
https://www.youtube.com/watch?v=7FZ6ZyzGex0

Another source of reference examples can be found here:
https://github.com/grpc-ecosystem/go-grpc-middleware