[metrics-team] Sudden drop in obfs3 users on 2016-09-02

[David Fifield]

In https://bugs.torproject.org/20216 we're investigating what looks like
a recent Tor block in Iran.

The userstats-combined graph appears to show an almost complete collapse
of obfs3 users on 2016-09-02 (which coincides with a decrease in direct
users):
https://metrics.torproject.org/userstats-bridge-combined.html?start=2016-06-24&end=2016-09-22&country=ir

However, Nima noticed that the same drop in obfs3 users appeared in
other countries on the same date:
https://metrics.torproject.org/userstats-bridge-combined.html?start=2016-06-24&end=2016-09-22&country=de
https://metrics.torproject.org/userstats-bridge-combined.html?start=2016-06-24&end=2016-09-22&country=ru
https://metrics.torproject.org/userstats-bridge-combined.html?start=2016-06-24&end=2016-09-22&country=us

I checked that the 5 current default obfs3 bridges are running, so
that's not the problem.

Did something change recently in the way bridge users are counted,
perhaps?