Replace set_var usage in examples (#829)

* examples: Fix inconsistent toml formatting

* examples: Replace set_var usage

examples/low-level-rustls/src/main.rs

@@ -18,14 +18,16 @@ use tokio_rustls::{
     TlsAcceptor,
 };
 use tower::MakeService;
+use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt};
 
 #[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", "example_tls_rustls=debug")
-    }
-    tracing_subscriber::fmt::init();
+    tracing_subscriber::registry()
+        .with(tracing_subscriber::EnvFilter::new(
+            std::env::var("RUST_LOG").unwrap_or_else(|_| "example_tls_rustls=debug".into()),
+        ))
+        .with(tracing_subscriber::fmt::layer())
+        .init();
 
     let rustls_config = rustls_server_config(
         "examples/tls-rustls/self_signed_certs/key.pem",