feat: Native axum executor integration

[yulnr]

Summary

Closes #62

Implements the new axum::serve::Executor on TelemetryHandle behind features = ["axum"]
Deletes axum_traced.rs (previous hand-rolled serve loop) and simplifies metrics-service example to use axum::serve directly.

Depends on this being released: tokio-rs/axum#3704

Before

Users had to reimplement axum's serve loop to inject traced task spawning:

// axum_traced.rs: custom Serve, WithGracefulShutdown,
// TracedExecutor, IncomingStream, accept loop, graceful shutdown...
axum_traced::serve(listener, app, handle.clone())
    .with_graceful_shutdown(signal)
    .await;

After

#[derive(Clone)]                                                                                                                                                                                      
struct TelemetryExecutor(TelemetryHandle);                       
                                            
impl axum::serve::Executor for TelemetryExecutor {
    fn execute<Fut>(&self, fut: Fut)
    where                                                                                                                                                                                             
        Fut: Future<Output = ()> + Send + 'static,
    {                                                                                                                                                                                                 
        self.0.spawn(fut);                                       
    }
}                                                                                                                                                                                                     
 
axum::serve(listener, app)                                                                                                                                                                            
    .with_executor(TelemetryExecutor(handle))                    
    .with_graceful_shutdown(signal)
    .await;

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[yulnr]

this unwrap diff is unrelated, unwrap no longer needed due to API change upstream

[yulnr]

Alternatively we could newtype this or have a struct that has the handle as field and impl axum::serve::Executor for it. This seemed the simplest for users.

[rcoh]

I'd probably rather have this as the 10 lines of code we need in application crates rather than putting it in core for now.

e.g. if we have multiple versions of axum etc. etc. this just becomes kind a headache.

[yulnr]

Good idea, done