What?

there is a new binary field for 'priority', and this should be supported by UPS

Why?

APNs offers priority changes, to indicate that the APNs server should attempt to deliver a notification "at a time that conserves power on the device receiving it."

How?

Leveraging https://github.com/relayrides/pushy/blob/5f82627d3a96ff0adabad91516c7fae2c088a853/pushy/src/main/java/com/relayrides/pushy/apns/DeliveryPriority.java class

Also, making sure it's compatible with the existing FCM priority:
https://github.com/aerogear/aerogear-unifiedpush-server/commit/abb78635298bf91a4639da84a163c491193917f7