[question] how to set header when a request leaves the grpc server

Hi devs,

Thanks a lot for developing tonic for rust grpc.

I'm looking for a solution of `tonic` to set header in an interceptor when a request leaves the grpc server, is there a helper for this to do so?

In golang, it can be written as
```
func fooInterceptor(ctx context.Context, req interface{}, info *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (interface{}, error) {
startTime := time.Now()
err := handler(ctx, req)
// elapsedTime := time.Since(startTime).Nanoseconds()
// set header
return err
}
```