Change mixed prometheus_client and probe_metric approachs to use PrometheusPusher

[voetberg]

Most probes in common use a combination of prometheus_client.CollectorRegistry/Gauge and probe_metrics, which is outdated now that PrometheusPusher exists and works.

The probes changed in pr #89 are also impacted, but so are all of the probes in common, if they use any context manager at all. It's easy enough to just update them all.