Fix auto-discovery for latest versions on Kubernetes

[L3n41c]

What does this PR do?

Leverages the possible_prometheus_urls parameter introduced by #9573 in the auto-discovery configuration file.

Motivation

Make the agent able to automatically monitor the Kube scheduler on most recent versions of Kubernetes without requiring too much user setup.
The goad is to simplify the Kubernetes control plane monitoring setup described in DataDog/documentation#10828.

Additional Notes

On older versions of Kubernetes, the Kube scheduler metrics were available at http://%%host%%:10251/metrics where %%host%% is the node IP.
On most recent version of Kubernetes,

• only the secure https endpoint is available by default. The plain http endpoint is disabled by default.
• the port is bound on the loopback interface on 127.0.0.1 instead of being remotely available.

The new auto-discovery configuration will try the combination of different settings to find a usable Kube scheduler metrics endpoint.
The only thing that the end-user will still need to do to get this check working is:

• either enabling hostNetwork: true on its agent to be able to use the 127.0.0.1 IP, or
• to configure its control plane components to expose their metrics endpoint on 0.0.0.0 as already described in K8s control plane docs documentation#10828.

Kubernetes exposes in the pods only the certificates needed to talk to the API server.
There is no reliable way to get the CA certificate used by the Kube scheduler that would work on every distribution. In particular, at least minikube and kubeadm are using self-signed certificates.
That’s why ssl_verify: false is required.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[codecov]

Codecov Report

Merging #9574 (7c4d6ea) into master (e97eb15) will not change coverage.
The diff coverage is n/a.

Flag	Coverage Δ
kube_scheduler	98.07% <ø> (ø)

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