[CONTP-947] Support openmetrics_endpoint in kube_scheduler check

[zhuminyi]

What does this PR do?

This PR adds support for the openmetrics_endpoint configuration field in the kube_scheduler, making them compatible with deployments that use this field name instead of prometheus_url.

Motivation

clusters using openmetrics_endpoint for kube_scheduler check are experiencing initialization failures with error message:

Error: {
  "Core Check Loader": "Check kube_scheduler not found in Catalog",
  "Python Check Loader": "could not configure check instance for python check kube_scheduler: could not invoke 'kube_scheduler' python check constructor. New constructor API returned:\nTraceback (most recent call last):\n  File 
  ...
  datadog_checks.base.errors.CheckException: Unable to find prometheus URL in config file.\nDeprecated constructor API returned:\nKubeSchedulerCheck.__init__() got an unexpected keyword argument 'agentConfig'"
}

Investigation showed that clusters are using pod annotations with openmetrics_endpoint field:

  {
    "kube_scheduler": {
      "instances": [{
        "openmetrics_endpoint": "https://%%host%%:10251/metrics",  
      }]
    }
  }

This PR implement OpenMetrics v2 support using the dual-mode pattern:

1. If only openmetrics_endpoint is provided, it's mapped to prometheus_url
2. If only prometheus_url is provided, it works as before
3. If neither field is provided, a clear error is raised
4. Both field names are accepted for maximum compatibility

With this priority setup:
New users see openmetrics_endpoint first (Priority 1) in documentation/examples
Existing users can continue using prometheus_url without changes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.96%. Comparing base (642a5e8) to head (cdcb1f3).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

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[github-actions]

⚠️ The qa/skip-qa label has been added with shippable changes

The following files, which will be shipped with the agent, were modified in this PR and
the qa/skip-qa label has been added.

You can ignore this if you are sure the changes in this PR do not require QA. Otherwise, consider removing the label.

List of modified files that will be shipped with the agent

kube_scheduler/datadog_checks/kube_scheduler/config_models/defaults.py
kube_scheduler/datadog_checks/kube_scheduler/config_models/instance.py
kube_scheduler/datadog_checks/kube_scheduler/config_models/validators.py
kube_scheduler/datadog_checks/kube_scheduler/data/conf.yaml.example

[github-actions]

⚠️ The qa/skip-qa label has been added with shippable changes

The following files, which will be shipped with the agent, were modified in this PR and
the qa/skip-qa label has been added.

You can ignore this if you are sure the changes in this PR do not require QA. Otherwise, consider removing the label.

List of modified files that will be shipped with the agent

kube_scheduler/changelog.d/21178.added
kube_scheduler/datadog_checks/kube_scheduler/config_models/defaults.py
kube_scheduler/datadog_checks/kube_scheduler/config_models/instance.py
kube_scheduler/datadog_checks/kube_scheduler/config_models/validators.py
kube_scheduler/datadog_checks/kube_scheduler/data/conf.yaml.example