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
```

roles/vmagent/files/ca-certs/kube-root-ca.crt

@@ -0,0 +1 @@
+../../../../kube-root-ca.crt