void track(String host, int port) {
      notificationsReceived.incrementAndGet();
      if (hosts.putIfAbsent(getKey(host, port), true) == null) {
        latch.countDown();
      }
    }
Tracks the event that arrived on {@code host}:{@code port}.
@param host the host that received the event
@param port the port of the host that describes the instance along with {@code host}.
ambry-test-utils/src/main/java/com/github/ambry/server/MockCluster.java