Container Registry: fix notification events delivered stat panel
As detailed in gitlab-org/container-registry#845 (comment 1342233002), there is a big discrepancy between the number of events delivered seen in the stat panel vs querying the counter directly in Thanos.
Overall, we had a total of 33,148/hour events delivered to Rails (source). Looking at the notification rate metrics (source), I see a total of 32,026/hour, which represents a deviation of -3%. This is small enough to be negligible. However, the metrics on the Grafana dashboard for this time period don't look good (e.g. only 1049 events delivered, when the underlying metric yields 32,026). So this needs to be fixed: