fix: slightly stricter mutual exclusivity constraints for metric name sharing

Revert to stricter mutual exclusivity constraints for metric name sharing with respect to labeled and unlabeled cases while still allowing different label sets per metric name. While Prometheus wouldn't break with sharing a metric name for labeled and unlabeled variants, the client enforces that each unique metric name must either be labeled (allowing multiple label sets) or unlabeled in order to support correct Prometheus aggregations.

Use MetricFamily instead of MetricGroup.

Signed-off-by: Melissa Kilby <[email protected]>

Author: incertum