kube-prometheus: Fix rule label selector

jsonnet/prometheus/prometheus-k8s-rules.libsonnet

@@ -4,5 +4,6 @@ local configMap = k.core.v1.configMap;
 {
     new(namespace, ruleFiles)::
         configMap.new("prometheus-k8s-rules", ruleFiles) +
+          configMap.mixin.metadata.withLabels({role: "alert-rules", prometheus: "k8s"}) +
           configMap.mixin.metadata.withNamespace(namespace)
 }