WireTap

Fan-out the stream to two output streams - a 'main' and a 'tap' one. Each incoming element is emitted to the 'main' output; elements are also emitted to the 'tap' output if there is demand; otherwise they are dropped.

'''Emits when''' element is available and demand exists from the 'main' output; the element will also be sent to the 'tap' output if there is demand.

'''Backpressures when''' the 'main' output backpressures

'''Completes when''' upstream completes

'''Cancels when''' the 'main' output cancels