code: def register_led_updates(self, callback, button):
          
          button_callbacks = self._button_observer_callbacks.get(button)
          if not button_callbacks:
              self._button_observer_callbacks[button] = []
          _LOGGER.debug('New callback for button %d', button)
          self._button_observer_callbacks[button].append(callback)
docs: Register a callback when a specific button LED changes.