Add transceiver metrics support

• Adds support for transceiver metrics (ethtool -m <device>)
• Adds an Info metric for basic stats (ethtool <device>)
• Adds alarms if any of the transceiver metrics is above or below threshold. This metric doesn't exist if everything is within limits.

Alarms are untested as of yet. If you have an ethtool -m output for a device that shows an alarm, please create an issue and paste it there.