hotpath: add measure_all to RingSink Sink impl

Reveals RingSink operation costs:
  poll_ready: 13ns/call (0.57% total)
  start_send: <26ns/call (below top-15 threshold)
  poll_flush: below top-15 threshold

Confirms ring tunnel sink operations are ~33ns/pkt total,
vs 2.04us for MpscTunnelSender::send (which routes through
tokio mpsc channel). The channel intermediary adds ~2us/pkt
of pure overhead.

easytier/src/tunnel/ring.rs

@@ -130,6 +130,7 @@ impl RingSink {
     }
 }
 
+#[cfg_attr(feature = "hotpath", hotpath::measure_all)]
 impl Sink<SinkItem> for RingSink {
     type Error = SinkError;