Proton Mail contacts → Android system address book.
A GPL-3.0 Android app that signs in to a Proton Mail account, decrypts the user’s contacts client-side, and exposes them to ContactsContract so they appear in the system Contacts app (and any other app that reads contacts) the same way WhatsApp or Telegram surface their own contact directories.
Leaving the encryption part aside, are you sure that uploading your Proton contacts directly to Google is something you want?