There are no metrics in SCRUM. This doesn't mean you can't have metrics, it just means that SCRUM doesn't mandate them nor tells you what you should do in this case, hence why you aren't finding anything of the sort.
Since we're talking about SCRUM, one usual mindset to this is to ask ourselves:
- is it useful? Does this
<activity,metric,task>
have a big value? - why do i want to do this? What will i get from it?
So instead of looking for the answer, i suggest you answer yourself that.
(PS: my boss asked me to is not the greatest motivational answer)