Extensible logstreams

/area API
/area test-and-release
/kind feature

I'd like to be able to extend the logstream (in Serving) to scrape things in the `serving-tests` namespace, e.g. to dump the `queue-proxy` logs for relevant resources.

cc @markusthoemmes @vagababov @coryrc

Start by locating the Serving logstream implementation and its handling of the serving-tests namespace. Trace how logs are collected for relevant resources, then determine what is needed for queue-proxy logs; done means the logstream can scrape those logs without breaking existing behavior.

Written by the indexing model from the issue text.