grafana-mixin: Fix expression for GrafanaRequestsFailing alert (#63382)

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

grafana-mixin/alerts/alerts.libsonnet

@@ -11,8 +11,8 @@
           {
             alert: 'GrafanaRequestsFailing',
             expr: |||
-              100 * namespace_job_handler_statuscode:grafana_http_request_duration_seconds_count:rate5m{handler!~"/api/datasources/proxy/:id.*|/api/ds/query|/api/tsdb/query", status_code=~"5.."}
-              / ignoring (status_code) group_left
+              100 * sum without (status_code) (namespace_job_handler_statuscode:grafana_http_request_duration_seconds_count:rate5m{handler!~"/api/datasources/proxy/:id.*|/api/ds/query|/api/tsdb/query", status_code=~"5.."})
+              /
               sum without (status_code) (namespace_job_handler_statuscode:grafana_http_request_duration_seconds_count:rate5m{handler!~"/api/datasources/proxy/:id.*|/api/ds/query|/api/tsdb/query"})
               > %(grafanaRequestsFailingThresholdPercent)s
             ||| % $._config,