message: add logger access and clear functions
diff: @@ -594,6 +594,19 @@ public:
  }
  }
  + /**
  + * Returns the vector containing all loggers registered at this object.
  + *
  + * @return the vector containing all registered loggers.
  + */
  + const std::vector<std::shared_ptr<const Logger>> &get_loggers() const
  + {
  + return loggers_;
  + }
  +
  + /** Remove all loggers registered at this object. */
  + void clear_loggers() { loggers_.clear(); }
  +
  protected:
  template <size_type Event, typename... Params>
  void log(Params &&... params) const
  