Get raw POST data from Firebase Function/Express/Node.js

Question

Using a Firebase Function to server POST requests. I need it to deal with application/xml data. But all I get back from req.body is an empty object {} as Firebase Functions appears to only support certain data payloads:

• application/json
• application/octet-stream
• text/plain
• application/x-www-form-urlencoded

So how can I get this work? I know Firebase Functions use Express underneath. Is there a way to get access to the raw POST data?