metrics: Scrape metrics from Kubernetes API server

Kubernetes exports a *lot* of metrics in Prometheus format.  I am not
sure what all is there, yet, but apparently several thousand time series
were added.

To allow anonymous access to the metrics, I added this RoleBinding:

```yaml
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: prometheus
rules:
- apiGroups:
  - ""
  resources:
  - nodes/metrics
  verbs:
  - get
- nonResourceURLs:
  - /metrics
  verbs:
  - get
```

group_vars/metricspi/scrape.yml

@@ -225,3 +225,11 @@ vmagent_scrape_configs:
   static_configs:
   - targets:
     - burp.pyrocufflink.blue:9000
+
+- job_name: kubernetes
+  scheme: https
+  tls_config:
+    ca_file: /etc/victoria-metrics/kube-root-ca.crt
+  static_configs:
+  - targets:
+    - kubernetes.pyrocufflink.blue:6443