smartctl_devices metric (and others) are often incorrect

[aieri]

In a standard server with a hardware HBA, the devices count is generally incorrect. This then causes the missing devices count to also be wrong. This issue is visible in the default documentation screenshot, which was pulled from a server in our lab that has 4 block devices (thankfully the screenshot itself isn't necessarily wrong - it could be displaying the correct output of a different server).

This is due to prometheus-community/smartctl_exporter#236; we could work around this by performing some level of hardware autodetection in the charm layer and configuring the exporter appropriately. Switching to a different exporter is also potentially an option.

[aieri]

additionally, I don't think the way we calculate missing devices makes sense, that difference should always be 0. We should consider dropping the panel.