apollo-biz/src/main/java/com/ctrip/framework/apollo/biz/entity/ReleaseHistory.java/ReleaseHistory/toString
class ReleaseHistory:  public String toString() {
    return toStringHelper().add("appId", appId).add("clusterName", clusterName)
        .add("namespaceName", namespaceName).add("branchName", branchName)
        .add("releaseId", releaseId).add("previousReleaseId", previousReleaseId)
        .add("operation", operation).toString();
  }