Stabilize the Monitoring class

[daniil-berg]

The Monitoring class is currently poorly documented, not particularly clear, nor type safe, and needlessly exposes some of its internal attributes.

It also currently stores simple "attribute-metrics" (that map to status attributes) and as well as "calculated metrics" (that require a function call in each update cycle) in the same metrics dictionary.

It would also benefit from the new Singleton metaclass to get rid of the get_instance class method.

Instead of referring to the global settings object throughout the class, it would make more sense to have the Prometheus text file path as a constructor argument that defaults to the corresponding setting. Referring to the storage base URL is probably fine as is.