I'm not actually sure that this gives you the light levels, but the most detailed command that I've been able to find for individual interfaces is 'show system state filter-pretty sys.sx.py.stats' where X is the slot number and Y is the port number.
I would check with your SE if there is an active feature request to add this feature into PAN-OS. If there is post it here so that others can add their votes to it ;-)
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