Use env!("CARGO_CRATE_NAME") in the example to simplify the tracing setup code (#2884)

examples/customize-extractor-error/src/main.rs

@@ -16,7 +16,7 @@ async fn main() {
     tracing_subscriber::registry()
         .with(
             tracing_subscriber::EnvFilter::try_from_default_env()
-                .unwrap_or_else(|_| "example_customize_extractor_error=trace".into()),
+                .unwrap_or_else(|_| format!("{}=trace", env!("CARGO_CRATE_NAME")).into()),
         )
         .with(tracing_subscriber::fmt::layer())
         .init();