I checked and indeed those counters didn't exist way way back in 7.3 :-). Originally I was working with 8.1 as a minimum, but then someone asked for 8.0 so I backported and did basic testing, and then the same happened for 7.3 and I must not have tested thoroughly enough on a system that had LUNs. In any case I have removed these counters from the default template so it will be fixed in the next Harvest release and you can also remove it manually now to stop the log spam messages:
2) Find the section with 'lun' counters and remove avg_read_latency avg_write_latency from the list.
3) Save the file and restart the impacted pollers.
For rotating of logfiles this is not something I have implemented yet. My original thought was if the logs are growing quickly the admin should fix the cause! But I agree, there should be some native rotation to avoid disk full so I will add this to the to-do list.
Cheers, Chris Madden
Storage Architect, NetApp EMEA (and author of Harvest)