Replace set_var usage in examples (#829)

* examples: Fix inconsistent toml formatting

* examples: Replace set_var usage

examples/unix-domain-socket/src/main.rs

@@ -28,6 +28,7 @@ use tokio::{
     net::{unix::UCred, UnixListener, UnixStream},
 };
 use tower::BoxError;
+use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt};
 
 #[cfg(not(unix))]
 fn main() {
@@ -37,11 +38,12 @@ fn main() {
 #[cfg(unix)]
 #[tokio::main]
 async fn main() {
-    // Set the RUST_LOG, if it hasn't been explicitly defined
-    if std::env::var_os("RUST_LOG").is_none() {
-        std::env::set_var("RUST_LOG", "debug")
-    }
-    tracing_subscriber::fmt::init();
+    tracing_subscriber::registry()
+        .with(tracing_subscriber::EnvFilter::new(
+            std::env::var("RUST_LOG").unwrap_or_else(|_| "debug".into()),
+        ))
+        .with(tracing_subscriber::fmt::layer())
+        .init();
 
     let path = PathBuf::from("/tmp/axum/helloworld");