Expose formatted service name as a const fn

[mkatychev]

Motivation

It is oftentimes cumbersome to use tonic::server::NamedService::NAME when T has multiple generics:

impl<T: ServerTrait> tonic::server::NamedService for ServiceServer<T> {
    const NAME: &'static str = "service_api.Service";
}

This is particularly egregious when using the HealthReporter methods since HealthReporter::set_serving requires a fully qualified ServiceServer<T>:

let (mut health_reporter, health_service) = tonic_health::server::health_reporter();
health_reporter.set_serving::<MyServer<MyInner<MyLongQualifiedType, MyOtherQualifiedType, T, T2>>>().await;

Solution

expose a pub const fn service_name() -> &'static str that returns the formatted string.