I'm not sure about this LogObserver api. Is this used a lot? I feel that those logs events are very raw (i.e not line oriented), and I cannot see use case, where custom steps would really need to have the logs as they happen.
So I would also mark this api deprecated, and recommend to use event + data api, if really needed.

or, if you really care about it, I would rather reimplement it in event + data. I feel that this api is a non generic method of registering to events that already exist (or will exist very soon).