[plugins] select perfdata inside the math() method of the --extend-perfdata-group option

Hello,

Here is an example of the need with the following perfdatas on a service:

cpu_total=60% cpu_1=20% cpu_2=40% 

I’d like to create a perfdata that is going to be the result of a substraction between two of the above metrics. Meaning that i want a new metric called cpu_custom that will be equal to cpu_total - cpu_1

To do so, I was thinking of doing the below option configuration

 --extend-perfdata-group=’cpu_total|cpu_1,cpu_custom,math(cpu_total - cpu_1)’

Sadly, it is not possible to do the math operation on metrics that are part of the searching group (cpu_total and cpu_1 according to my regex). 

I’d like to have a way to handle that a bit like the sum() method where you can use a regex to filter metrics from your searching group and then do the sum. 

Regards