Add a datasource variable $DS_PROMETHEUS

[pauvos]

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]

Hi @pauvos. 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.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[aledbf]

/ok-to-test
/lgtm

[aledbf]

@pauvos thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, pauvos

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [aledbf]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[codecov-io]

Codecov Report

Merging #4797 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4797      +/-   ##
==========================================
- Coverage   58.59%   58.57%   -0.02%     
==========================================
  Files          88       88              
  Lines        6702     6702              
==========================================
- Hits         3927     3926       -1     
- Misses       2349     2351       +2     
+ Partials      426      425       -1

Impacted Files	Coverage Δ
internal/watch/file_watcher.go	80.76% <0%> (-3.85%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 46953cc...f5a02c1. Read the comment docs.

[aledbf]

/retest