fix: remove duplicate #[must_use] attribute on with_label (#16)

The with_label trait method had two #[must_use] attributes, causing
a compiler warning. Kept the more descriptive message version.

Author: refcell

crates/traits/src/runtime.rs

@@ -56,7 +56,6 @@ pub trait Spawner: Clone + Send + Sync + 'static {
         T: Send + 'static;
 
     /// Spawn with a label for metrics/tracing.
-    #[must_use]
     #[must_use = "with_label returns a new labeled spawner, the original is unchanged"]
     fn with_label(&self, label: &str) -> Self;