code: public static String newMultiTaskId(String jobId, int sequence) {
      return Id.MultiTask.create(Id.parse(jobId).get(Id.Parts.INSTANCE_NAME), sequence).toString();
    }
docs: Create an ID for a new multi-task (corresponding to a {@link org.apache.gobblin.source.workunit.MultiWorkUnit})
  for the job with the given job ID.
  
  @param jobId job ID
  @param sequence multi-task sequence number
  @return new multi-task ID