New function: growth

[widhalmt]

Hi,

Could you add a function in addition to sum and average that allows for giving thresholds for growth?

A trivial solution could be: threshold (given in percent of starting value) for ending value - starting value. I'm sure there are better solutions.

I just ran some tests with your plugins for integration into the Icinga 2 book and came up with the idea. I'd be happy to help with work on this issue when I have a bit of spare time.

Cheers, Thomas