Expose zdb reachability as a Prometheus metric

[scottyeager]

Currently the status of whether a given zdb is reachable is not exposed as a Prometheus metric. Given #118 it's also not possible to rely on the presence of other metrics as an indication of whether the zdbs are reachable, though I think that's not ideal anyway.

Let's add an explicit metric for each metadata and data backend zdb the indicates whether zstor is able to reach it.

[scottyeager]

Duplicate of #72.