add metrics on throttled streams (#34579)

(cherry picked from commit 1a00175)

streamer/src/nonblocking/quic.rs

@@ -742,6 +742,7 @@ async fn handle_connection(
                     if reset_throttling_params_if_needed(&mut last_throttling_instant) {
                         streams_in_current_interval = 0;
                     } else if streams_in_current_interval >= max_streams_per_100ms {
+                        stats.throttled_streams.fetch_add(1, Ordering::Relaxed);
                         let _ = stream.stop(VarInt::from_u32(STREAM_STOP_CODE_THROTTLING));
                         continue;
                     }

streamer/src/quic.rs

@@ -156,6 +156,7 @@ pub struct StreamStats {
     pub(crate) connection_setup_error_locally_closed: AtomicUsize,
     pub(crate) connection_removed: AtomicUsize,
     pub(crate) connection_remove_failed: AtomicUsize,
+    pub(crate) throttled_streams: AtomicUsize,
 }
 
 impl StreamStats {
@@ -386,6 +387,11 @@ impl StreamStats {
                 self.total_stream_read_timeouts.swap(0, Ordering::Relaxed),
                 i64
             ),
+            (
+                "throttled_streams",
+                self.throttled_streams.swap(0, Ordering::Relaxed),
+                i64
+            ),
         );
     }
 }