Determines from which key in an event the attribute will be populated.
The value can be either:
- known analytics event data key (e.g.
analytics.source_medium)
- custom event data key (
^[a-z0-9_]{1,255}$, e.g. user_score) - can be used to update attributes via API
If an attribute with the given event_data_key already exists, its type must match;
otherwise, an appropriate error will be returned.