internal/etl: add trace debugging for TestETL
internal/etl: add trace debugging for TestETL

A custom trace exporter is added that prints spans to the test log. This
can be used to debug test timing using existing trace instrumentation.

Updates b/142715549

Change-Id: Ia12ca1ff97bdd4b1536bbe850adc347418a953c0
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/573876
CI-Result: Cloud Build <ea5729d4cf3392b9d382b6ca272c65aa27554f44@system.gserviceaccount.com>
Reviewed-by: Julie Qiu <e6cb64861edcc38971590ff27c111e0e672de045@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite