How to limit different concurrency number by service on Tonic?

[0xbillw]

If we want to add the concurrency limit on the service on the server side, we can do this:

let layer = tower::ServiceBuilder::new().
    .load_shed()
    .concurrency_limit(2)
    .into_inner();

Service::builder()
    .layer(layer)
    .add_service(greeter)
    .add_service(echo)
    .serve(addr)
    .await?;

The above code we have: the greeter and echo service maximum concurrency is 2, both for the connection of concurrent cap of a total of 4, exert the function of each layer service is the same, I understand it right?

Now what if I want to implement different concurrency limits for the greeter and echo services, say 3 for one and 2 for the other? I couldn't find a corresponding method in Tonic's documentation or code.
However, for the Tower Service it is possible, I can wrap the two services separately, for example:

let greeter = tower::ServiceBuilder::new()
    .load_shed()    
    .concurrency_limit(3)
    .service(greeter);

let echo = tower::ServiceBuilder::new()
    .load_shed()    
    .concurrency_limit(2)
    .service(echo);

Server::builder()
    .add_service(greeter)
    .add_service(echo)
    .serve(addr)
    .await?;

But that won't work. Because add_service() method requires the participation of type must implement the tonic::server::NamedService trait and the Error association type must be Infallible.
How to solve this problem?
Do you have similar needs when using Tonic?
Additionally, I also asked this question on stackoverflow.