How to declare type S for into_service ?

```rust 
let grpc_service: S = transport::Server::builder()
    .add_service(service1)
    .add_service(service2)
    .add_service(service3)
    .into_service();
```

How to declare type S?

I had try ` tonic::server::RouterService<A>`, but i can not find generic type in multiple services mode  for  type A.
