kube-prometheus: generate manifests without kubectl

For `--dry-run` to work with kubectl a Kubernetes cluster's apiserver is
actually used, which is unnecessary for generating these manifests. This
approach also allows further customization, such as adding labels to the
generated manifests.

assets/prometheus/rules/etcd2.rules

@@ -29,7 +29,7 @@ ALERT HighNumberOfFailedHTTPRequests
 
 # alert if more than 5% of requests to an HTTP endpoint have failed with a non 4xx response
 ALERT HighNumberOfFailedHTTPRequests
-  IF sum by(method) (rate(etcd_http_failed_total{job="etcd-k8s", code!~"4[0-9]{2}"}[5m])) 
+  IF sum by(method) (rate(etcd_http_failed_total{job="etcd-k8s", code!~"4[0-9]{2}"}[5m]))
     / sum by(method) (rate(etcd_http_received_total{job="etcd-k8s"}[5m])) > 0.05
   FOR 5m
   LABELS {