fix: trace delayed websocket acknowledgements

loadtest/im-pressure/callback_queue_test.go

@@ -45,3 +45,20 @@ func TestWaitForCallbackQueueWaitsForProcessingCallback(t *testing.T) {
 		t.Fatal("wait returned before the callback finished")
 	}
 }
+
+func TestWaitForDeliveryQuiescenceResetsForLateDelivery(t *testing.T) {
+	recorder := newDeliveryRecorder("run", "a", 1)
+	queue := &callbackQueueMetrics{}
+	go func() {
+		time.Sleep(20 * time.Millisecond)
+		recorder.received.Add(1)
+	}()
+
+	started := time.Now()
+	if err := waitForDeliveryQuiescence(context.Background(), recorder, queue, 150*time.Millisecond, time.Second); err != nil {
+		t.Fatal(err)
+	}
+	if elapsed := time.Since(started); elapsed < 250*time.Millisecond {
+		t.Fatalf("quiet period was not reset after late delivery: elapsed=%s", elapsed)
+	}
+}