apollo-biz/src/main/java/com/ctrip/framework/apollo/biz/grayReleaseRule/GrayReleaseRulesHolder.java/GrayReleaseRulesHolder/assembleGrayReleaseRuleKey
class GrayReleaseRulesHolder:  private String assembleGrayReleaseRuleKey(String configAppId, String configCluster, String
      configNamespaceName) {
    return STRING_JOINER.join(configAppId, configCluster, configNamespaceName);
  }