Fix High number of 500 errors alert to work instance-wide

This was originally intentioned to be look at the instance-wide stats,
but I have accidentally copied the wrong query from my experiments.

service_rules.yml

@@ -10,7 +10,7 @@ groups:
     annotations:
       summary: "{{ $labels.instance }} has a high rate of 500 errors on  route {{ $labels.route }}"
   - alert: High rate of 500 errors on an instance
-    expr: rate(request_time_count{status="500"}[15m]) / ignoring(status) sum without(status) (rate(request_time_count[15m])) > 0.25
+    expr: sum by(instance) (rate(request_time_count{status="500"}[15m])) / sum by(instance) (rate(request_time_count[15m])) > 0.25
     for: 5m
     labels:
       severity: urgent