Google Hangouts Chat API IP to Whitelist?

Question

I am building a bot that will interact with the Google Hangouts Chat API. I need to punch a hole in my firewall to receive incoming HTTPS requests.

Is there a subnet I can expect Google to send requests from? I would rather not open the bot to the entire world.

For organizational reasons, we cannot use pub/sub endpoints. We just need an ip or ip range from google to whitelist