According to a correspondent for the Android development team on the Google Issue Tracker, a future Android release will allow users to start a manual app data backup to Google Drive. This includes backing up your application data, call history, device settings, and text messages. As it stands, your backups go to Google Drive automatically under certain conditions like time, power status, and more, without the possibility of user intervention through normal means. Thankfully, users will be able to trigger the backup themselves in a future Android release so that you can change phones with the knowledge your data is safely stored in the cloud.

It's not the case, however, that you can't trigger a backup manually at all. The functionality does exist, but you'll need to set up ADB on your computer by enabling USB debugging on your Android smartphone. You can then use the following command on your computer to trigger a manual backup of all your data onto Google Drive. 

        adb shell bmgr backupnow --all

While you can switch on and off auto-backup by going to Settings -> System -> Backup & Reset on an Android Pie device like the Google Pixel 2 XL, the only real way to trigger a backup manually is via the ADB command above.

Google Pixel 2 XL Android Pie backup
Settings --> System --> Backup settings on the Google Pixel 2 XL running Android 9 Pie

Once Google adds this feature to Android, you won't have to rely on an obscure ADB command anymore. It's not going to be the most useful of features for the average consumer, but it will certainly be used by some. Fortunately, apps backed up via the built-in Android backup manager do not use up your Google Drive's storage quota. Application-specific backups such as WhatsApp do count against your storage space when backing up, though that's set to change for the hugely popular messaging client. We'll be keeping an eye out for this feature to go live, and we're hopeful that it could potentially launch with the next iteration of Android Pie.