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 :)
Actually a good question. I'm not sure when they get reset. If they get reset on each boot, it would be rather a gauge, no?