Variable based on a new metric's label from prometheus DS

If I add a new label to all prometheus metrics used in a panel and create a Grafana variable around it, obviously old metrics won’t have this newly-added label. How can I use the new variable in panels, while continuing to show old series that didn’t contain said new label?

Just using a label=~'$variable' in query won’t do, if there is no label in the first place.
So ideally, I’d like to show all data points if there is no label, and only those filtered by label if said label exists.

To answer my own question, .* for custom all values in variable definition seems to do the trick.