apollo-configservice/src/main/java/com/ctrip/framework/apollo/configservice/util/InstanceConfigAuditUtil.java/InstanceConfigAuditUtil/assembleInstanceKey
class InstanceConfigAuditUtil:  private String assembleInstanceKey(String appId, String cluster, String ip, String datacenter) {
    List<String> keyParts = Lists.newArrayList(appId, cluster, ip);
    if (!Strings.isNullOrEmpty(datacenter)) {
      keyParts.add(datacenter);
    }
    return STRING_JOINER.join(keyParts);
  }