[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.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[jichenjc]

/ok-to-test

[Lucasgranet]

Hello,

Have you any feedback or questions about this PR?

[Lucasgranet]

Hello, @jichenjc @dulek,

If you have some time, would you mind reviewing this PR please?

Thank you,

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[k8s-ci-robot]

PR needs rebase.

Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes-sigs/prow](https://github.com/kubernetes-sigs/prow/issues/new?title=Prow%20issue:) repository.