tracking authenticated Drupal traffic in Varnish

As part of the performance and load testing suite I am building for a client, they asked that it accurately reflect the percentage of traffic which is logged in. Sure, no problem, we told them. But how do we get this? Really, their setup provided no easy way to collect this info (multi-site with ~200 sites). The Google Analytics module *could* be used, as it provides custom variables and we could then use a token with the users role. But, that would require building out a feature, deploying it, and enabling.

Caching pages in Varnish with HTTP Basic Authentication

Normally, if Varnish sees that a page has HTTP Basic Auth on it, it will immediately trigger a pass to the backend. This was causing some performance issues with the staging setup for one of our clients. They had a (smaller) mirrored setup of their production systems, just for staging. Varnish -> 2 Apache backends -> 1 DB server. But, all their staging sites have HTTP Basic Authentication on them, to keep the public's prying eyes away. Our sys-admin discovered this article, and we are working on implementing this as a solution for them right now.