feat(telemetry): integrate OpenTelemetry observability stack with health metrics

- Add OpenTelemetry SDK, OTLP exporter, Prometheus integration
- Implement connection tracking with active/total/disconnection metrics
- Add health endpoint with uptime and connection counts
- Integrate tracing spans for socket events and engine messages
- Add metrics collection for event handling duration
- Update health endpoint to include live runtime state
- Add graceful telemetry shutdown in main function
- Implement engine session active metrics tracking
- Add namespace-specific attributes to connection metrics
- Introduce message edit history retrieval endpoint
- Add scheduled message CRUD operations and dispatcher
- Update Socket.IO event registration with observability
- Refactor component update to remove dead code allowance
- Add comprehensive environment variables documentation
- Implement detailed development guidelines in AGENTS.md

engine/server.rs

@@ -115,17 +115,26 @@ impl EngineServer {
         ));
         let heartbeat_handle = heartbeat.start();
 
-        tracing::info!("Engine.IO HTTP server listening on {}", addr);
+        tracing::info!(
+            endpoint = %addr,
+            "Engine.IO HTTP server listening, /health and /metrics available"
+        );
 
         let result = HttpServer::new(move || {
             App::new()
                 .app_data(web::Data::new(store.clone()))
                 .app_data(web::Data::new(config.clone()))
                 .app_data(web::Data::new(on_message.clone()))
+                // Health check with connection metrics
                 .route(
                     "/health",
                     web::get().to(crate::engine::health::health_check),
                 )
+                // Prometheus metrics endpoint
+                .route(
+                    "/metrics",
+                    web::get().to(crate::telemetry::metrics::metrics_handler),
+                )
                 .route("/engine.io/", web::get().to(engine_get))
                 .route(
                     "/engine.io/",