apollo-client-config-data/src/main/java/com/ctrip/framework/apollo/config/data/internals/PureApolloConfig.java/PureApolloConfig/getPropertyNames
class PureApolloConfig:  @Override
  public Set<String> getPropertyNames() {
    // pure apollo config only contains the property from repository and the property from additional
    Set<String> fromRepository = this.getPropertyNamesFromRepository();
    Set<String> fromAdditional = this.getPropertyNamesFromAdditional();
    if (CollectionUtils.isEmpty(fromRepository)) {
      return fromAdditional;
    }
    if (CollectionUtils.isEmpty(fromAdditional)) {
      return fromRepository;
    }
    Set<String> propertyNames = Sets
        .newLinkedHashSetWithExpectedSize(fromRepository.size() + fromAdditional.size());
    propertyNames.addAll(fromRepository);
    propertyNames.addAll(fromAdditional);
    return propertyNames;
  }