This bug is for implementing the "durations" field in the baseline ping, "The combined duration of all the foreground sessions since the last upload".

I don't think this would be super informative if we keep it a "timespan". The definition says "The combined duration of all the foreground sessions since the last upload". But we're sending the baseline ping every time we go to background. So this will end up measuring, every single time, the duration of the last foreground session. We should either make this a timing distribution or have an "application" lifetime.