observe
Watches a key and reports every change to listener. Registering the same listener for a key twice is ignored.
The first listener for a key subscribes to it in the background, so a change made before that completes is not reported. A failed subscription goes to RoomEventListener.onError, with 4085 when the server refuses it.
Parameters
key
the key to watch
listener
receives the new value on each change
Throws
if key is empty