docs: public final ZMonitor add(Event... events)
      {
          if (started) {
              System.out.println("ZMonitor: Unable to add events while already started.");
              return this;
          }
          ZMsg msg = new ZMsg();
          msg.add(ADD_EVENTS);
          for (Event evt : events) {
              msg.add(evt.name());
          }
          agent.send(msg);
          agent.recv();
          return this;
      }