StatsD: only report medians to Ganglia

This patch prevents aggregate figures that aren't median from being reported to
Ganglia. We do care about the other aggregate figures, but they will remain
available in Graphite, and are not vital for rendering the Navigation Timing
view on Ganglia (whereas the medians are).

Change-Id: I7078bf230198f71b11babdd89b331190a8e895bf