Looking at our logs, it seems they don't go back far enough (we have "only" 3 years of logs by the look of it), and I estimate this was set up / potentially deleted more like 4-5 years ago (but somehow report builder can still use the metric)
You can check in here for changes made to Calculated Metrics, you can even further reduce this list by adding the name, or even a partial name of the metric that is missing.