feat(ethtool): query SFP module diagnostics from EEPROM

[tdakkota]

Motivation

SFP transceivers expose live diagnostic data through the Digital Diagnostics Monitoring (DDM) interface, accessible today via:

$ sudo ethtool -m enp5s0f1
        ...
	Laser bias current                        : 32.450 mA
	Laser output power                        : 0.4374 mW / -3.59 dBm
	Receiver signal average optical power     : 0.5784 mW / -2.38 dBm
	Module temperature                        : 41.55 degrees C / 106.78 degrees F
	Module voltage                            : 3.3715 V
        ...

It can provide crucial data by identifying failing transceivers and monitoring the thermal health of modules.

Spec links

There are few specifications describing encoding of SFP diagnostics data.

• https://members.snia.org/document/dl/25916
• https://members.snia.org/document/dl/26418