groupBy

Which event opens a new collapsible group in the viewers' timeline, and which of its properties labels that group. Defaults to Firebase's manual-screen_view convention, so grouping works without calling this; pass a null event to turn it off. When labelProperty is null, or the event doesn't carry it, the group takes the event's own name.

The decision is made once, at capture: the matching event is stamped and the viewers group by that stamp. Changing it later only affects events captured from then on.