Could we inject this retry into __iter__ somehow? this seems more generally useful to me.

Because the underlying API call is a unary_stream, the response_iterator is an instance of google.api_core.grpc_helpers._StreamingResponseIterator: I guess we could modify its __next__ to do something like this (it is already tweaked to cache the first result, for instance).

I don't see how we get the applicable retry object inside there -- the _wrapped attribute isn't our _GapicCallable, but something altogether different.