Incorporate heartbeat checks into prometheus metrics

Checkly's current prometheus endpoint exports metrics for api and browser checks.

However, it does not export anything related to heartbeat type checks.

To make it possible to use Checkly's Prometheus metrics for all types of checks, it would be good to expose metrics for heartbeat type checks too.

For inspiration, I think it's useful to look at what metrics kube-state-metrics export for cronjobs: https://github.com/kubernetes/kube-state-metrics/blob/main/docs/cronjob-metrics.md

Specifically, something similar to kube_cronjob_status_last_successful_time would be useful for alerting.

Alternatively, if you simply incorporate heartbeat type checks into the existing checkly_check_status metric, that'll work too.

Currently, our alerting stack is based heavily on Prometheus, and we are able to re-use our existing alerting stack for checkly api checks, but need to do something separate to handle heartbeat type checks. That's why we want this feature.