Prometheus - label metrics by region

Currently the Checkly Prometheus endpoint doesn’t include a region label in the metrics. Users may want to view their metrics by region, though.

There should be a configuration option to add a region label to the metrics. For users that don’t opt-in to this, though, the behavior can stay the same.

Including the region label will increase the cardinality of the metrics significantly (f.ex. checks running on 12 regions will have 12x as many metrics). This should be communicated in the documentation on the feature.