code: public boolean ack(String taskId, @Nullable String workerId) {
          Preconditions.checkArgument(StringUtils.isNotBlank(taskId), "Task id cannot be blank");
  
          TaskServicePb.AckTaskRequest.Builder request = TaskServicePb.AckTaskRequest.newBuilder();
          request.setTaskId(taskId);
          if (workerId != null) {
              request.setWorkerId(workerId);
          }
  
          return stub.ackTask(request.build()).getAck();
      }
docs: Ack for the task poll.
  
  @param taskId   Id of the task to be polled
  @param workerId user identified worker.
  @return true if the task was found with the given ID and acknowledged. False otherwise. If the server returns false, the client should NOT attempt to ack again.