Iterates the event's properties as strings. Note that keys may be repeated and a get property request may call your sink with null. It it does, it means the key either doesn't request or cannot be represented by json in the current implementation.

This is auto-implemented for you if you mixin RegisterGetters in your child classes and only have properties of type bool, int, double, or string. For other ones, you will need to do it yourself as of the June 2, 2021 release.