Fix expression for GrafanaRequestsFailing alert
The intent of the alert is to get the ratio of 5xx to all status codes
[^1]. With the original expression, the left hand side can have more than
one row with the same labels except for the status code. This results in
a promql error because it is doing a many-to-one matching. Doing a sum
on the left hand side first should preserve the intent of the alert and
resolve the issue.
[^1]: https://github.com/grafana/grafana/pull/43116
Due to PromQL's label matching, both sides of the division will have same series; that means that, whenever there's a 5xx error, both sides will have the same value and the division will be `1`. I believe the idea was to get the ratio of 5xx compared will all status code, and to do that, we need to aggregate the `status_code` dimension away.
