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

socket/server.rs

@@ -143,29 +143,40 @@ async fn handle_engine_message(
 ) {
     if let EnginePacketData::Text(ref text) = engine_packet.data {
         match parser::decode(text) {
-            Ok(socket_packet) => match socket_packet.packet_type {
-                PacketType::Connect => {
-                    handle_connect(
-                        &engine_sid,
-                        &socket_packet,
-                        namespaces,
-                        socket_txs,
-                        engine_store,
-                        adapter,
-                    )
-                    .await;
+            Ok(socket_packet) => {
+                let packet_type = format!("{:?}", socket_packet.packet_type);
+                let _span = tracing::debug_span!(
+                    "engine_message",
+                    engine_sid = %engine_sid,
+                    packet_type = %packet_type,
+                    namespace = %socket_packet.namespace,
+                );
+                let _enter = _span.enter();
+
+                match socket_packet.packet_type {
+                    PacketType::Connect => {
+                        handle_connect(
+                            &engine_sid,
+                            &socket_packet,
+                            namespaces,
+                            socket_txs,
+                            engine_store,
+                            adapter,
+                        )
+                        .await;
+                    }
+                    PacketType::Disconnect => {
+                        handle_disconnect(&engine_sid, &socket_packet, namespaces, socket_txs);
+                    }
+                    PacketType::Event => {
+                        handle_event(&engine_sid, &socket_packet, namespaces);
+                    }
+                    PacketType::Ack => {
+                        handle_ack(&engine_sid, &socket_packet);
+                    }
+                    _ => {}
                 }
-                PacketType::Disconnect => {
-                    handle_disconnect(&engine_sid, &socket_packet, namespaces, socket_txs);
-                }
-                PacketType::Event => {
-                    handle_event(&engine_sid, &socket_packet, namespaces);
-                }
-                PacketType::Ack => {
-                    handle_ack(&engine_sid, &socket_packet);
-                }
-                _ => {}
-            },
+            }
             Err(e) => {
                 tracing::warn!(engine_sid = %engine_sid, error = %e, "Invalid Socket.IO packet");
             }
@@ -181,6 +192,13 @@ async fn handle_connect(
     engine_store: &SessionStore,
     adapter: &Arc<dyn Adapter>,
 ) {
+    let _span = tracing::info_span!(
+        "socket_connect",
+        engine_sid = %engine_sid,
+        namespace = %packet.namespace,
+    );
+    let _enter = _span.enter();
+
     // Validate namespace path to prevent DoS via arbitrary namespace creation
     if !crate::socket::namespace::is_valid_namespace(&packet.namespace) {
         tracing::warn!(
@@ -244,11 +262,16 @@ async fn handle_connect(
                 break;
             }
         }
-        // Forwarding task ended — ensure socket is cleaned up from namespace
+        // Forwarding task ended — ensure socket is cleaned up from namespace.
+        // If the socket was still registered (session expiry / engine disconnect
+        // without Socket.IO disconnect packet), also update the connection counter.
         socket_txs_clone.remove(&socket_sid_clone);
-        namespace_clone
+        let was_removed = namespace_clone
             .remove_socket_by_sid(&socket_sid_clone)
             .await;
+        if was_removed {
+            crate::telemetry::health::connection_disconnected();
+        }
     });
 
     // Send Connect response (only after handler passed)
@@ -268,16 +291,26 @@ fn handle_disconnect(
     namespaces: &Arc<NamespaceManager>,
     socket_txs: &Arc<DashMap<String, mpsc::Sender<Packet>>>,
 ) {
-    if let Some(namespace) = namespaces.get_namespace(&packet.namespace) {
-        // Look up socket by engine_sid, then remove by socket_sid
-        if let Some(socket) = namespace.get_socket_by_engine_sid(engine_sid) {
-            socket_txs.remove(&socket.sid);
-            let socket_sid = socket.sid.clone();
-            let ns_clone = namespace.clone();
-            tokio::spawn(async move {
-                ns_clone.remove_socket_by_sid(&socket_sid).await;
-            });
-        }
+    if let Some(namespace) = namespaces.get_namespace(&packet.namespace)
+        && let Some(socket) = namespace.get_socket_by_engine_sid(engine_sid)
+    {
+        let m = crate::telemetry::metrics::get();
+        m.connections_active.add(
+            -1,
+            &crate::telemetry::MetricsInstruments::namespace_attrs(&socket.namespace),
+        );
+        m.disconnections_total.add(
+            1,
+            &crate::telemetry::MetricsInstruments::namespace_attrs(&socket.namespace),
+        );
+        crate::telemetry::health::connection_disconnected();
+
+        socket_txs.remove(&socket.sid);
+        let socket_sid = socket.sid.clone();
+        let ns_clone = namespace.clone();
+        tokio::spawn(async move {
+            ns_clone.remove_socket_by_sid(&socket_sid).await;
+        });
     }
 }