Seems like this would be more of a manual task, rather than automated.

Export data from Infusionsoft.

Analyze data in GSheets via formulas.

One option might be to explore creating a saved search for contacts added in the last week, have it emailed weekly, then parse the email for responses, and push to Sheets

I know you requested a monthly report but I think Infusionsoft can only be set up to email reports weekly. https://help.infusionsoft.com/help/search-by-date-created 

Another that is closer to what you are wanting is to use the “List Contacts” endpoint.

 https://developer.infusionsoft.com/docs/rest/#!/Contact/listContactsUsingGET 

order the above by the “date created” created field and then do some parsing of the response and voila! This would definitely be an advanced solution though. If you’re not comfortable implementing it yourself, I would love the opportunity to help build you a custom solution.