For each Scrum master and for the whole agile team the most important is the done selected backlogs for the sprint, but not time spent on coffee breaks or browsing the internet. And it’s the known fact, that in agile development the most essential issue is how much time is left to finish the user story, but not how much time was spent by each of the developer.
You should ask yourself whether you need the metrics of time at all. Why are you tracking time? One reasonable answer that comes to my mind can be to bill a Client: to know how much time was spent, to pay overtimes. If you can suggest more, I’ll appreciate your feedback.
Nevertheless there are some tips of tracking time: