Opens the stream.

def open(self):
        """Opens the stream."""
        if self.is_active:
            raise ValueError("Can not open an already open stream.")

        request_generator = _RequestQueueGenerator(
            self._request_queue, initial_request=self._initial_request
        )
        call = self._start_rpc(iter(request_generator), metadata=self._rpc_metadata)

        request_generator.call = call

        # TODO: api_core should expose the future interface for wrapped
        # callables as well.
        if hasattr(call, "_wrapped"):  # pragma: NO COVER
            call._wrapped.add_done_callback(self._on_call_done)
        else:
            call.add_done_callback(self._on_call_done)

        self._request_generator = request_generator
        self.call = call