code: public FileSystemCommand heartbeat(final long workerId,
        final List<Long> persistedFiles) throws IOException {
      return heartbeat(workerId, persistedFiles, FileSystemHeartbeatPOptions.newBuilder().build());
    }
docs: Heartbeats to the master. It also carries command for the worker to persist the given files.
  
  @param workerId the id of the worker that heartbeats
  @param persistedFiles the files which have been persisted since the last heartbeat
  @return the command for file system worker