[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" # 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.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[jichenjc]

/ok-to-test

[Lucasgranet]

/retest

[Lucasgranet]

/retest

[Lucasgranet]

/retest

[Lucasgranet]

/retest

[Lucasgranet]

/retest-required

[Lucasgranet]

/retest

[Lucasgranet]

/retest