Closes#25.

This PR adds the ability to disable automatically pre-fetching the first item of a stream returned by*-Stream gRPC callables. This hook will be used in PubSub to fix thestalled stream issue, while also not affecting Firestore, since the default behavior is preserved.

I realize the fix is far from ideal, but it's the least ugly among the approaches I tried, e.g. somehow passing the flag throughResumableBidiRpc (it's a messy rabbit hole). On the PubSub side monkeypatching the generated SubscriberClient will be needed, but it's a (relatively) clean one-liner:

diff --git google/cloud/pubsub_v1/gapic/subscriber_client.py google/cloud/pubsub_v1/gapic/subscriber_client.pyindex e98a686..1d6c058 100644--- google/cloud/pubsub_v1/gapic/subscriber_client.py+++ google/cloud/pubsub_v1/gapic/subscriber_client.py@@ -1169,6 +1169,8 @@ class SubscriberClient(object):                 default_timeout=self._method_configs["StreamingPull"].timeout,                 client_info=self._client_info,             )+            # TODO: explain this monkeypatch!+            self.transport.streaming_pull._prefetch_first_result_ = False          return self._inner_api_calls["streaming_pull"](             requests, retry=retry, timeout=timeout, metadata=metadata

If/when we merge this, we should also release it, and then we can add!= 1.17.0 to thegoogle-api-core version pin in PubSub.

PR checklist

• Make sure to open an issue as abug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)