PresenceDegradationReporter as a HealthMetricProvider

[bplattenburg]

We've previously built the Presence Degradation Reporter, which counts all the devices present in the local mesh, but there's no way to monitor & alert from it. Since then, we created the concept of the HealthMetricProvider to report on various things via the heartbeat.

We should be able to combine the two so that you can configure presence degradation (with or without a UI) as a HealthMetricProvider to include in your heartbeat

[zmarkan]

Another benefit would be to be able to capture some presence data without delays (and blurry snaps of dirty screens 😅)