A bit of self promotion, though not opensource. If you store your data in sheets, Probe Sheets plugin [1] calculates basic metrics based on some basic data: customer id, mrr, start and end time of subscription. We do not store the data you send (there is no way I can prove that to you though, but you can always obfuscate it somehow)
[1] https://workspace.google.com/marketplace/app/probe/108508041...