Enhancement for Invoke-IcingaCheckNetworkInterface error and discard checks

In my opinion the currently implemented checks for interface discards and errors are suboptimal at least when it comes to percentage thresholds which for me is the only useful threshold „unit“. In a network it is not unusual to have a few errors and discards. What really matters is if there are too much of them in propotion to the overall packets (= percent).
The problem with the current implementation is that the corresponding windows performance counters are incrementing values.
Let‘s say I have 100.00 received packets during a sample interval and 10 discards which means 0,01% discards.
Now let‘s say within the next sampling interval I receive 10 packets and 0 discards. But the check will show 100% discards because the incrementing windows discard counter is still 10.
So in my opinion it would make more sense to use a delta value for discards and errors for the percentage calculation.

Start by locating Invoke-IcingaCheckNetworkInterface and the handling of Windows performance counters for interface errors and discards. Reproduce successive sampling intervals with incrementing counters, then verify that percentage thresholds use interval deltas and remain meaningful when packet counts change.