void trackUpdate(String host, int port, UpdateType updateType) {
    updateHelpers.computeIfAbsent(updateType, type -> new Helper(numberOfReplicas)).track(host, port);
  }
Tracks the update event of type {@code updateType} that arrived on {@code host}:{@code port}.
@param host the host that received the update
@param port the port of the host that describes the instance along with {@code host}.
@param updateType the {@link UpdateType} received
ambry-test-utils/src/main/java/com/github/ambry/server/MockCluster.java