feat(client): add receive metadata scaffold

crates/irtt-client/src/client/tests/probes_replies.rs

@@ -284,6 +284,7 @@ fn recv_once_decodes_echo_reply_and_emits_event() {
             received_stats,
             server_timing,
             bytes,
+            packet_meta,
             ..
         } => {
             assert_eq!(*seq, 0);
@@ -298,6 +299,10 @@ fn recv_once_decodes_echo_reply_and_emits_event() {
             assert!(server_timing.is_some());
             let st = server_timing.as_ref().unwrap();
             assert!(st.processing.is_some());
+            assert_eq!(packet_meta.traffic_class, None);
+            assert_eq!(packet_meta.dscp, None);
+            assert_eq!(packet_meta.ecn, None);
+            assert_eq!(packet_meta.kernel_rx_timestamp, None);
         }
         other => panic!("expected EchoReply, got {other:?}"),
     }