Calculate the delta since the last time the methods were called for a given jamon monitor key.
Note any of the delta objects only reset numbers for the specific method being called.
This works out ok if the use case is to call all methods at the same time as would happen in jconsole.
In this case all methods are called and so all are reset at the same time. If only 1 method is called
then the deltas could get out of sync.