message: getter for max_history_length
diff: @@ -142,6 +142,10 @@ class Agent : public Object {
  return first_valid_timestamp_;
  }
  + int GetMaxHistoryLength() const {
  + return max_history_length_;
  + }
  +
  //! Setter
  void SetBehaviorModel(const BehaviorModelPtr& behavior_model_ptr) {
  behavior_model_ = behavior_model_ptr;
  