bridges (especially unpublished ones) should include usage info in their heartbeats
As part of SponsorJ task 2, we're running some fast unpublished bridges. Since they're unpublished, we have no way to learn how much usage they see. We should at least log them so the operators can send us the log snippets over time.
As a side benefit, logging will help the bridge operators who don't use Vidalia/arm and wonder whether their bridge is being helpful.