promhttp handlers panic with "metric partitioned with non-supported labels" when providing a ConstLabel #299
Description
The example for the promhttp handlers mention adding a ConstLabel {"handler": "some_handler"}.
When you do this, checkLabels will panic. I think it would make sense to not look at the ConstLabel when checking for labels. Otherwise you have to encode the handler name into the metric names to get sensible metrics for your handlers.