gddo-server: add debug logs

The log exports to BigQuery disappeared as of 1/15. Add additional debug
logs.

Change-Id: I6defe5adfa20bd6401bf30fdb5f6ebedde65ef0e
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/284934
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>