kube-vip: optional Prometheus metrics (default port 2112) (#13229)

Add kube_vip_metrics_enabled and kube_vip_metrics_port; wire
prometheus_server and container port in the static pod manifest.
Default metrics port to 2112 to match kube-vip upstream. Document
and sample inventory updated.

roles/kubernetes/node/templates/manifests/kube-vip.manifest.j2

@@ -109,11 +109,21 @@ spec:
 {% if kube_vip_lb_fwdmethod %}
     - name: lb_fwdmethod
       value: {{ kube_vip_lb_fwdmethod | string | to_json }}
+{% endif %}
+{% if kube_vip_metrics_enabled %}
+    - name: prometheus_server
+      value: {{ (':' ~ (kube_vip_metrics_port | string)) | to_json }}
 {% endif %}
     image: {{ kube_vip_image_repo }}:{{ kube_vip_image_tag }}
     imagePullPolicy: {{ k8s_image_pull_policy }}
     name: kube-vip
     resources: {}
+{% if kube_vip_metrics_enabled %}
+    ports:
+    - name: metrics
+      containerPort: {{ kube_vip_metrics_port }}
+      protocol: TCP
+{% endif %}
 {% if kube_vip_lb_fwdmethod == "masquerade" %}
     securityContext:
       privileged: true