chore: Fix simple example and doc

doc/draft.md

@@ -10,6 +10,12 @@ This example is meant to show basic service-based IMAP server with boitalettres
 $ cargo run --example simple
 ```
 
+If you want to trace this `simple` example with the [`tokio-console`](https://github.com/tokio-rs/console/tree/main/tokio-console) utility, run:
+
+```shell
+$ RUSTFLAGS="tokio_unstable" cargo run --example simple --features "tokio/tracing"
+```
+
 - [Basic python testing script](../scripts/test_imap.py)
 
 ```shell

examples/simple.rs

@@ -40,9 +40,7 @@ async fn main() -> Result<()> {
 fn setup_logging() {
     use tracing_subscriber::prelude::*;
 
-    tracing_subscriber::registry()
-        .with(console_subscriber::spawn())
-        .with(
+    let registry = tracing_subscriber::registry().with(
         tracing_subscriber::fmt::layer().with_filter(
             tracing_subscriber::filter::Targets::new()
                 .with_default(tracing::Level::DEBUG)
@@ -52,6 +50,10 @@ fn setup_logging() {
                 .with_target("tokio_tower", tracing::Level::TRACE)
                 .with_target("mio", tracing::Level::TRACE),
         ),
-        )
-        .init();
+    );
+
+    #[cfg(tokio_unstable)]
+    let registry = registry.with(console_subscriber::spawn());
+
+    registry.init();
 }