I can confirm that the domain does not appear to be captured in any field in the request_log request. The issue trackers that can likely host this request would be the
App Engine public issue tracker or the
Cloud Platform public issue tracker. Though Stackdriver Logging enhancements generally should be filed with the Product-CloudLogging component in the Cloud Platform tracker, logged in users and them being tracked in request logs seems more like an App Engine issue as we'd have to first confirm that the App Engine frontend (doing this logging) actually sends that piece of data to Stackdriver Logging. If it's accepted when we review the FR, we'll create an internal tracking number that will be directed to the appropriate engineering team in any case.
Once you've file this feature request to the
App Engine public issue tracker, please post a link to it here so that others in the community can support the request.