apollo-configservice/src/main/java/com/ctrip/framework/apollo/configservice/util/WatchKeysUtil.java/WatchKeysUtil/assembleWatchKeys
class WatchKeysUtil:  private Multimap<String, String> assembleWatchKeys(String appId, String clusterName,
                                                     Set<String> namespaces,
                                                     String dataCenter) {
    Multimap<String, String> watchedKeysMap = HashMultimap.create();

    for (String namespace : namespaces) {
      watchedKeysMap
          .putAll(namespace, assembleWatchKeys(appId, clusterName, namespace, dataCenter));
    }

    return watchedKeysMap;
  }