Track time for tags (not just projects)

I could see counting time in the summary/planning of the day, but not counting time spent on tags. This would be useful if, for example, our aim is to limit the time spent on certain tag activities f.e. mail, or to much meetings. I mean just would be good to cosider how to add possibility not but not too intrusive for UI and UX. Maybe just add a little touchdown showing this. Some sort of drop down list idk.