server, metrics: remove the connection count on server, only use the metrics (#51996)

[ti-chi-bot]

This is an automated cherry-pick of #51996

What problem does this PR solve?

Issue Number: close #51889

Problem Summary:

1. The connection count is not correct, if the session resets the resource group manually.
2. Maintaining a separated map on server to count the connection for each resource group seems to be not necessary. We can use the GaugeVec itself. The connection count is not used anywhere else.
3. The grafana panel of connection count is confusing. I added a legend and now it looks like:

Maybe someone prefers to sum up them together. Actually I think both are fine, feel free to comment your opinion.

Personally, I don't like the idea to use the uncontrollable string as a label 🤦, because the prometheus will have much greater pressure when these labels multiply together.

As for now, it seems not a big issue, but it'll become much worse if anyone adds more similar labels.

What changed and how does it work?

1. Use the GaugeVec as the only counter of the connection.
2. Also increase and decrease the counter when the user set resource group manually.
3. Modify the grafana definition. I found that some modification on the json file is not represented in the jsonnet, so I also fix them.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Release note

Fix the issue that the connection count is not calculated and shown correctly.

[codecov]

Codecov Report

Attention: Patch coverage is 98.83721% with 1 lines in your changes are missing coverage. Please review.

Please upload report for BASE (release-7.5@1cc3f9b). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-7.5     #53056   +/-   ##
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  Coverage               ?   72.0054%           
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  Files                  ?       1411           
  Lines                  ?     410234           
  Branches               ?          0           
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  Hits                   ?     295391           
  Misses                 ?      94984           
  Partials               ?      19859           

Flag	Coverage Δ
unit	72.0054% <98.8372%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	53.9913% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	53.4109% <0.0000%> (?)

[YangKeao]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tiancaiamao, YangKeao

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-05-22 07:34:14.626476586 +0000 UTC m=+2243408.383612151: ☑️ agreed by tiancaiamao.
• 2024-05-22 07:34:50.446954573 +0000 UTC m=+2243444.204090146: ☑️ agreed by YangKeao.