kube_apiserver_metrics: add example config for Datadog Operator

[keisku]

What does this PR do?

add example config.

Motivation

Hard to configure configmap with Datadog Operator by the current doc.

Additional Notes

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [ ] Changelog entries must be created for modifications to shipped code
• [ ] Add the qa/skip-qa label if the PR doesn't need to be tested during QA.