[metrics] implement labels with a default value

What does this MR do?

I originally made this for !1572 (merged) but during the review we realized that there was an easier option using a new metric.

That being said, the preparatory work to add a default value to a label can still be handy in the future to handle backward compatible label expansion.

To ensure backward-compatibility we can now define labels with a default value. This must be a temporary solution to give us time to upgrade components.

This is especially useful when the metric is sent from a coordinated pipeline that is running from an older commit.