Some metrics that accumulate daily quantities send their samples on the day change, using this code:

  const TimeDelta since_epoch = now_wall_time - Time::UnixEpoch();
  const int day = since_epoch.InDays();
  const int week = day / 7;

so all devices in the same time zone will send a sample approximately at the same time.  This should be spread randomly instead, especially if we switch all devices to UTC (see issue 889227).  The new CumulativeMetrics class already does that, and it would make sense to use that consistently.