As Michael indicates OnCommand Unified Manager certainly will meet the need for the Filer alert and most every other Filer reporting/alerting need you may have. If you are using Nagios to monitor a bunch of discrete hardware/systems and you just want to add in this item, there is not an SNMP counter in the NetApp MIB that directly provides what you want. All the NFS statistics are "since last reset", so you have two basic options to make this work.
1. Get counter values over time and derive the rate in your collection tool based on the differences.
2. Get counter values and immediately reset the statistics via "nfsstat -z" as a command to the Filer. Due to the natural lag between query and reset there will be some loss of signal in this mechanism of course.
I've found that most all collected statistics available are every increasing counters as they are efficient to implement at the Filer, leaving any rate/per type calculations to the management tool's resources.