Start second server on different port for metrics endpoint (#1444)

* Start second server on different port for metrics endpoint

* Don't track metrics on metrics server

1 files changed, 39 insertions, 14 deletions

diff --git a/examples/prometheus-metrics/src/main.rs b/examples/prometheus-metrics/src/main.rs
index 1152b3c7..5ab3fbdd 100644
--- a/examples/prometheus-metrics/src/main.rs
+++ b/examples/prometheus-metrics/src/main.rs
@@ -23,19 +23,13 @@ use std::{
 };
 use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt};
 
-#[tokio::main]
-async fn main() {
-    tracing_subscriber::registry()
-        .with(tracing_subscriber::EnvFilter::new(
-            std::env::var("RUST_LOG")
-                .unwrap_or_else(|_| "example_todos=debug,tower_http=debug".into()),
-        ))
-        .with(tracing_subscriber::fmt::layer())
-        .init();
-
+fn metrics_app() -> Router {
     let recorder_handle = setup_metrics_recorder();
+    Router::new().route("/metrics", get(move || ready(recorder_handle.render())))
+}
 
-    let app = Router::new()
+fn main_app() -> Router {
+    Router::new()
         .route("/fast", get(|| async {}))
         .route(
             "/slow",
@@ -43,15 +37,46 @@ async fn main() {
                 tokio::time::sleep(Duration::from_secs(1)).await;
             }),
         )
-        .route("/metrics", get(move || ready(recorder_handle.render())))
-        .route_layer(middleware::from_fn(track_metrics));
+        .route_layer(middleware::from_fn(track_metrics))
+}
+
+async fn start_main_server() {
+    let app = main_app();
 
     let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
     tracing::debug!("listening on {}", addr);
     axum::Server::bind(&addr)
         .serve(app.into_make_service())
         .await
-        .unwrap();
+        .unwrap()
+}
+
+async fn start_metrics_server() {
+    let app = metrics_app();
+
+    // NOTE: expose metrics enpoint on a different port
+    let addr = SocketAddr::from(([127, 0, 0, 1], 3001));
+    tracing::debug!("listening on {}", addr);
+    axum::Server::bind(&addr)
+        .serve(app.into_make_service())
+        .await
+        .unwrap()
+}
+
+#[tokio::main]
+async fn main() {
+    tracing_subscriber::registry()
+        .with(tracing_subscriber::EnvFilter::new(
+            std::env::var("RUST_LOG")
+                .unwrap_or_else(|_| "example_todos=debug,tower_http=debug".into()),
+        ))
+        .with(tracing_subscriber::fmt::layer())
+        .init();
+
+    // The `/metrics` endpoint should not be publicly available. If behind a reverse proxy, this
+    // can be achieved by rejecting requests to `/metrics`. In this example, a second server is
+    // started on another port to expose `/metrics`.
+    let (_main_server, _metrics_server) = tokio::join!(start_main_server(), start_metrics_server());
 }
 
 fn setup_metrics_recorder() -> PrometheusHandle {