'A callback that is invoked when any RPC completes.
If specified, it will be called with a UserRPC object as argument when an
RPC completes.
NOTE: There is a subtle but important difference between
UserRPC.callback and Configuration.on_completion: on_completion is
called with the RPC object as its first argument, where callback is
called without arguments.  (Because a Configuration\'s on_completion
function can be used with many UserRPC objects, it would be awkward
if it was called without passing the specific RPC.)'

@ConfigOption
def on_completion(value):

    return value
