void trackPurge(String host, int port) {
    purgeHelper.track(host, port);
  }
Tracks the purge event that arrived on {@code host}:{@code port}.
@param host the host that received the purge request.
@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