feat: add Prometheus metrics endpoint and query instrumentation

src/handler/health.rs

@@ -1,11 +1,23 @@
+use axum::extract::State;
 use axum::http::StatusCode;
 use axum::routing::get;
 use axum::Router;
+use sea_orm::{ConnectionTrait, Statement};
 
 use crate::state::AppState;
 
-async fn healthz() -> StatusCode {
-    StatusCode::OK
+async fn healthz(State(state): State<AppState>) -> StatusCode {
+    match state
+        .db
+        .execute(Statement::from_string(
+            sea_orm::DatabaseBackend::Sqlite,
+            "SELECT 1".to_string(),
+        ))
+        .await
+    {
+        Ok(_) => StatusCode::OK,
+        Err(_) => StatusCode::SERVICE_UNAVAILABLE,
+    }
 }
 
 pub fn router() -> Router<AppState> {

src/handler/metrics.rs

@@ -0,0 +1,13 @@
+use axum::extract::State;
+use axum::routing::get;
+use axum::Router;
+
+use crate::state::AppState;
+
+async fn metrics(State(state): State<AppState>) -> String {
+    state.metrics_handle.render()
+}
+
+pub fn router() -> Router<AppState> {
+    Router::new().route("/metrics", get(metrics))
+}

src/handler/mod.rs

@@ -2,6 +2,7 @@ pub mod domains;
 mod health;
 mod host_meta;
 pub mod links;
+mod metrics;
 pub mod tokens;
 mod webfinger;
 
@@ -45,5 +46,6 @@ pub fn router(state: AppState) -> Router {
         .merge(public_routes)
         .merge(api_routes)
         .merge(health::router())
+        .merge(metrics::router())
         .with_state(state)
 }

src/handler/webfinger.rs

@@ -35,15 +35,36 @@ async fn webfinger(
     State(state): State<AppState>,
     uri: Uri,
 ) -> AppResult<Response> {
+    let start = std::time::Instant::now();
+
     let (resource_opt, rels) = parse_webfinger_query(&uri);
 
     let resource = resource_opt
         .ok_or_else(|| AppError::BadRequest("missing resource parameter".into()))?;
 
-    let cached = state
-        .cache
-        .get(&resource)
-        .ok_or(AppError::NotFound)?;
+    // Extract domain from resource for metrics labeling
+    let resource_domain = resource
+        .split('@')
+        .nth(1)
+        .or_else(|| {
+            // Handle URI-style resources like https://domain/path
+            resource.split("://").nth(1).and_then(|s| s.split('/').next())
+        })
+        .unwrap_or("unknown")
+        .to_string();
+
+    let cached = match state.cache.get(&resource) {
+        Some(c) => {
+            metrics::counter!("webfinger_queries_total", "domain" => resource_domain.clone(), "status" => "hit").increment(1);
+            c
+        }
+        None => {
+            metrics::counter!("webfinger_queries_total", "domain" => resource_domain.clone(), "status" => "miss").increment(1);
+            let elapsed = start.elapsed().as_secs_f64();
+            metrics::histogram!("webfinger_query_duration_seconds").record(elapsed);
+            return Err(AppError::NotFound);
+        }
+    };
 
     let links: Vec<serde_json::Value> = cached
         .links
@@ -94,6 +115,9 @@ async fn webfinger(
 
     response_body.insert("links".into(), json!(links));
 
+    let elapsed = start.elapsed().as_secs_f64();
+    metrics::histogram!("webfinger_query_duration_seconds").record(elapsed);
+
     Ok((
         [
             (header::CONTENT_TYPE, "application/jrd+json"),

src/state.rs

@@ -1,3 +1,4 @@
+use metrics_exporter_prometheus::PrometheusHandle;
 use sea_orm::DatabaseConnection;
 use std::sync::Arc;
 
@@ -11,4 +12,5 @@ pub struct AppState {
     pub cache: Cache,
     pub settings: Arc<Settings>,
     pub challenge_verifier: Arc<dyn ChallengeVerifier>,
+    pub metrics_handle: PrometheusHandle,
 }

tests/common/mod.rs

@@ -1,3 +1,4 @@
+use metrics_exporter_prometheus::PrometheusBuilder;
 use sea_orm::{ConnectOptions, ConnectionTrait, Database, DatabaseConnection, Statement};
 use sea_orm_migration::MigratorTrait;
 use std::sync::Arc;
@@ -57,10 +58,20 @@ pub async fn test_state_with_settings(settings: Settings) -> AppState {
     let db = setup_test_db().await;
     let cache = Cache::new();
     cache.hydrate(&db).await.unwrap();
+    // Each test gets its own metrics recorder. If install_recorder fails because
+    // another test already installed one, build a standalone recorder and grab its handle.
+    let metrics_handle = PrometheusBuilder::new()
+        .install_recorder()
+        .unwrap_or_else(|_| {
+            let recorder = PrometheusBuilder::new().build_recorder();
+            recorder.handle()
+        });
+
     AppState {
         db,
         cache,
         settings: Arc::new(settings),
         challenge_verifier: Arc::new(webfingerd::challenge::MockChallengeVerifier),
+        metrics_handle,
     }
 }