Fix TargetDown alert text

Currently, when TargetDown fires, we get messages like

> 0% of the envoy-stats targets are down.

which is obviously wrong.

The fix is to change the value of the metric. Rather than averaging
`up` to get a percentage of instances that are up, we average `1-up`
to get a percentage of instances that are down.

charts/gsp-cluster/charts/gsp-monitoring/templates/rules-general.yaml

@@ -9,7 +9,7 @@ spec:
     - alert: TargetDown
       annotations:
         message: '{{`{{ $value }}`}}% of the {{`{{ $labels.job }}`}} targets are down.'
-      expr: 100 * (avg without(instance, pod) (up)) < 90
+      expr: 100 * (avg without(instance, pod) (1 - up)) > 10
       for: 10m
       labels:
         severity: warning