Fix grafana datasources

[vasrem]

What this PR does / why we need it:

We need to assign the correct datasources in the grafana dashboard.

Special notes for your reviewer:

I have submitted two different commits. The first commit (2a3f03c) solves the problem of using default datasource which doesn't make sense in my opinion since in other parts of the file we use prometheus (like here). This leads to wrong data inputs and in addition to wrong visualization of the nginx metrics.

The second commit d1a38c2 fixes a bug which shows on the right corner of grafana the following message.

Annotation Query Failed
Datasource named Prometheus was not found

FYI: I'm using the following setup:
Grafana v5.2.1
Prometheus v2.3.1

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[vasrem]

/check-cla

[vasrem]

/assign @caseydavenport

[aledbf]

@vasrem please don't assign PRs manually

[aledbf]

@vasrem please add a comment about the Grafana version (something like v5.2.0 or higher is required) in https://github.com/kubernetes/ingress-nginx/blob/master/deploy/grafana/dashboards/README.md

[k8s-ci-robot]

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[vasrem]

I think its ok now.

[codecov-io]

Codecov Report

Merging #2888 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2888      +/-   ##
==========================================
- Coverage   47.56%   47.54%   -0.02%     
==========================================
  Files          76       76              
  Lines        5483     5483              
==========================================
- Hits         2608     2607       -1     
- Misses       2540     2542       +2     
+ Partials      335      334       -1

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

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[aledbf]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[aledbf]

@vasrem thanks!

[kerneljack]

@aledbf @vasrem Thanks for noting that version 5.2.0 of Grafana is required. Can you please let me know which feature from that version is required for this dashboard? I'm currently on 5.1.3 and am thinking of upgrading.

[vasrem]

@kerneljack Could you test if this feature works with 5.1.3? I did some tests for several previous versions and I saw that it works for >=5.2.0. I'm not sure which feature is required since I didn't delve into the code.

[kerneljack]

@vasrem unfortunately I've already upgraded to 5.2.x now, so I can no longer test it for you, apologies :-(

[kerneljack]

@vasrem btw, when you say test if this feature works with 5.1.3, what specific feature are you referring to?

[vasrem]

@kerneljack I couldn't reply earlier. My mistake. Regarding the feature, I meant the dashboard yaml that I modified in that PR.

[kerneljack]

@vasrem no worries, it's ok, it works for us now anyway in 5.2.x so it's good, thanks for fixing it :-)

[kerneljack]

@vasrem btw, do you have a list of upcoming features or panels that you are planning on adding to that dashboard? For example, we prefer to see the error rates on a line graph "per-upstream" instead of the table that you have. We just happen to find that a lot more intuitive so I was wondering if you're going to add anything like that at some point.