Somehow, the JSON printer seems to have never emitted correct summary
statistics. And I believe #3178 is the first time anyone has ever
reported it. I believe this bug has persisted for years. That's
surprising.
Anyway, the problem here was that we were bailing out of `finish()` on
the sink if we weren't supposed to print anything. But we bailed out
before we tallied our summary statistics. Obviously we shouldn't do
that.
Fixes#3178
63209ae0b9