Replace set_var usage in examples (#829)

* examples: Fix inconsistent toml formatting

* examples: Replace set_var usage

examples/static-file-server/src/main.rs

@@ -7,17 +7,17 @@
 use axum::{http::StatusCode, routing::get_service, Router};
 use std::net::SocketAddr;
 use tower_http::{services::ServeDir, trace::TraceLayer};
+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_static_file_server=debug,tower_http=debug",
-        )
-    }
-    tracing_subscriber::fmt::init();
+    tracing_subscriber::registry()
+        .with(tracing_subscriber::EnvFilter::new(
+            std::env::var("RUST_LOG")
+                .unwrap_or_else(|_| "example_static_file_server=debug,tower_http=debug".into()),
+        ))
+        .with(tracing_subscriber::fmt::layer())
+        .init();
 
     let app = Router::new()
         .nest(