Add MegaCLI collector

This collector exports the following metrics:

- raid_drive_temperature: drive temperature
- raid_drive_count: drive error and event counters
- raid_adapter_disk_presence: disk presence per adapter

I still have to see if everything is working as expected, but feel free to review already :)
Not really. If resets are the only downwards movement, I'd say it's a counter. I guess what you could ask yourself is: "does it make sense to do `rate()` over it?". If yes, then it should probably be a counter.