How to get only the first occurrence of a log line by userId?

Hey hey,
I am logging the call of a rest API in a private System and i want to find out, when it was called by a specific user for the first time.
I have the following labels: UserId - timestamp of the call - customerId - payload

So what i need is a distinct min function of the timestamp. GroupBy in combination with min was looking good, but it seems there is no way to not remove the other labels in the result. I want to display the result in a table panel.

Is this even possible in Loki? Any suggestions are welcome.

Thank you.