fix(metrics): correct frontend and backend protocol connection tracking across h1, h2, h3, and websocket traffic

rust/crates/rustproxy-http/src/h3_service.rs

@@ -18,7 +18,7 @@ use tracing::{debug, warn};
 use rustproxy_config::RouteConfig;
 use tokio_util::sync::CancellationToken;
 
-use crate::proxy_service::{ConnActivity, HttpProxyService};
+use crate::proxy_service::{ConnActivity, HttpProxyService, ProtocolGuard};
 
 /// HTTP/3 proxy service.
 ///
@@ -48,6 +48,9 @@ impl H3ProxyService {
         let remote_addr = real_client_addr.unwrap_or_else(|| connection.remote_address());
         debug!("HTTP/3 connection from {} on port {}", remote_addr, port);
 
+        // Track frontend H3 connection for the QUIC connection's lifetime.
+        let _frontend_h3_guard = ProtocolGuard::frontend(Arc::clone(self.http_proxy.metrics()), "h3");
+
         let mut h3_conn: h3::server::Connection<h3_quinn::Connection, Bytes> =
             h3::server::builder()
                 .send_grease(false)