apollo-client/src/main/java/com/ctrip/framework/apollo/internals/DefaultConfig.java/DefaultConfig/stringPropertyNames
class DefaultConfig:  private Set<String> stringPropertyNames(Properties properties) {
    //jdk9以下版本Properties#enumerateStringProperties方法存在性能问题，keys() + get(k) 重复迭代, jdk9之后改为entrySet遍历.
    Map<String, String> h = Maps.newLinkedHashMapWithExpectedSize(properties.size());
    for (Map.Entry<Object, Object> e : properties.entrySet()) {
      Object k = e.getKey();
      Object v = e.getValue();
      if (k instanceof String && v instanceof String) {
        h.put((String) k, (String) v);
      }
    }
    return h.keySet();
  }