[occm] Support Octavia/Amphora Prometheus endpoint creation using annotations

[Lucasgranet]

What this PR does / why we need it:
Adds the ability to add a Prometheus listener on the Octavia LoadBalancer in order to fetch it with any Prometheus scrapper to get metrics from the LoadBalancer.

This PR brings 4 new annotations:

loadbalancer.openstack.org/metrics-enable: "true" # Enable the listener endpoint on the Octavia LoadBalancer (default false)
loadbalancer.openstack.org/metrics-port: "9101" # Listener's port (default 9100)
loadbalancer.openstack.org/metrics-allow-cidrs: "10.0.0.0/8, fe80::/10" # Listener's allowed cidrs (default none) - see below
loadbalancer.openstack.org/load-balancer-vip-address: "10.4.2.3" #  Auto-computed field based on Octavia VIP

Why you should have to use loadbalancer.openstack.org/metrics-allow-cidrs

It's strongly recommended to apply an allowed cidrs on the listener. If a Floating IP is attached to the Octavia, the metric endpoint will be exposed publicly. Apply a restriction to the K8S's subnet is recommended.

More detail on the Prometheus listener:
https://docs.openstack.org/octavia/latest/user/guides/monitoring.html#monitoring-with-prometheus

Which issue this PR fixes(if applicable):
fixes #2465

Special notes for reviewers:
A new E2E test has been added.

Release note:

[openstack-cloud-controller-manager] Support Octavia/Amphora Prometheus endpoint creation using annotations

[k8s-ci-robot]

Hi @Lucasgranet. Thanks for your PR.

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[jichenjc]

/ok-to-test

[Lucasgranet]

/retest

[k8s-ci-robot]

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• ✅ login: Lucasgranet / name: Lucas Granet (1f83d4c, 5ed72af)

[k8s-ci-robot]

PR needs rebase.

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