code: public void setFileMetadata(String file, String key, Object val) {
      throwIfImmutable();
      Map<String, Object> fileKeys = fileLevel.get(file);
      if (fileKeys == null) {
        fileKeys = new ConcurrentHashMap<>();
        fileLevel.put(file, fileKeys);
      }
  
      fileKeys.put(key, val);
      cachedId = null;
    }
docs: Set an arbitrary file-level metadata key