Is there a way to extend already defined PTransforms? My question is probably better illustrated with an example. Let's say I have a PTransform that generates a very variable number of outputs. I'd like to "wrap" that PTransform such that if it ever creates more than say 1,000 outputs, then I just take the first 1,000 outputs without generating the rest of the outputs.
It'd be trivial if I have access to the DoFn, but what if the PTransform in question doesn't expose the `DoFn`?
