UnboundedReader exposes method getTotalBacklogBytes() which promises: * Returns the size of the backlog of unread data in the underlying data source represented by split of this source.
But there are no implementations of this method in any source, I'm assuming that this is because splits are instance of the same UnboundedSource impl class, which can be distributed to different JVMs or machines, and there isn't a context within the source class in which all generated splits' backlog can be summed. If this is the case, what is the use of this method in the API?
