ExperimentalOptionalgetExperimentalReturns event properties to merge into the final user action event. Only
keys prefixed with event_properties. are accepted. Properties set
directly on the user action take precedence over tracker properties with
the same key. Called exactly once when the user action completes, before
stop().
Rich handle returned by a creator that contributes
event_properties.*to the final user action event. For trackers that don't contribute properties, the creator can return the stop function directly instead of this object.