As an Engineering in the team building Wikibase REST API I would like to know what errors happen in production environments so that I can understand what requests led to an error and reason about those cases.
Errors on Wikidata are all sent to WMF's Logstash: https://logstash.wikimedia.org/
Having a way to quickly see and further filter all errors that happen on test.wikidata.org and www.wikidata.org in requests to REST API would likely suffice for the purpose of that story