def observe(observable, persistent: true)
      # register for observable updates, and set initial value
      if value = observable.add_observer(self, persistent: persistent)
        debug { "observe #{observable} => #{value}" }

        add(observable, value)
      else
        debug { "observe #{observable}..." }

        add(observable)
      end
    end
def observe(observable, persistent: true)
      # register for observable updates, and set initial value
      if value = observable.add_observer(self, persistent: persistent)
        debug { "observe #{observable} => #{value}" }

        add(observable, value)
      else
        debug { "observe #{observable}..." }

        add(observable)
      end
    end
Observe observable: add Observer to Observable, and add Observable to Observer.

 NOTE: Must be called from exclusive mode, to ensure that any resulting Observable messages are nost lost before calling receive!

 @param observable [Kontena::Observable]
 @param persistent [Boolean] false => only interested in current or initial value