If I send a request like this via Measurement Protocol:
v=1&tid=UA-123456-2&cid=455646456.4646456456&uid=123
What will happen? Given that UID hasn't been set on the site before.
Will this request associate the provided client ID with user ID 123
?
Or can I only provide a UID that has already been set on the website like this:
ga('set', 'userId', USER_ID);