Add DS_PROMETHEUS datasource

[pauvos]

Add a datasource variable $DS_PROMETHEUS

What this PR does / why we need it: Current grafana dashboard does not work with dashboard provisioning (e.g. the default prometheus-operator setup) due to missing $DS_PROMETHEUS datasource.

Which issue this PR fixes: fixes #4190

Special notes for your reviewer:

[k8s-ci-robot]

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The list of commits with invalid commit messages:

• 35ab8c2 Add DS_PROMETHEUS datasource

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: pauvos
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[pauvos]

Will open up another one because of invalid commit message...