Fix metric names (2) - Change dots to underscores

Created by: hacklschorsch

As soon as our custom metrics turn up (see #91 (closed)) Prometheus stops parsing all of our metrics. Turns out it wasn't the CAPS but the dots (.) in the metric names that gave us the problems all along.

This fixes https://whetstone.privatestorage.io/privatestorage/PrivateStorageio/-/issues/78 and is related to #86 (closed) .