server: refine the connection counter logic

[YangKeao]

What problem does this PR solve?

Issue Number: close #54545, close #54428

Problem Summary:

This PR solve two problems:

1. The inc/dec of the connection doesn't organize in pair, so the connection count may decrease below zero. It actually increases in registerConn, and decreases in conn.Close() (if the conn.getCtx() is not nil). If the connection exists before handshake successfully, for example authentication failed, the total connection count will decrease one 🤦.
2. SET SESSION_STATES (usually used by tiproxy) didn't INC/DEC the connection count for the resource group if it's changed.

What changed and how does it work?

This PR reorganized the codes INC/DEC the metric gauge in the following pattern:

1. It's increased in newConn with default resource group name, so that it can make sure the connection count will always increase.
2. It's decreased in closeConn. If the connection doesn't have a valid context, it'll decrease in the default resource group.

Other use case of the ConnGauge always comes in pair: increase one and decrease another one. This PR provides a method SetResourceGroupName to change the session vars and always INC/DEC the metric counter. It'll be called in SET SESSION_STATES, SET RESOURCE GROUP and authentication (because the user may carry a resource group info).

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 correct if some connection exits before handshake finishes.
Fix the issue that with tiproxy and non-default resource group, the connection count of each resource group is not correct.

[codecov]

Codecov Report

Attention: Patch coverage is 97.97980% with 2 lines in your changes missing coverage. Please review.

Project coverage is 57.1109%. Comparing base (761b592) to head (ee804f1).
Report is 142 commits behind head on master.

Additional details and impacted files

@@                Coverage Diff                @@
##             master     #54546         +/-   ##
=================================================
- Coverage   72.8839%   57.1109%   -15.7731%     
=================================================
  Files          1523       1672        +149     
  Lines        435584     629739     +194155     
=================================================
+ Hits         317471     359650      +42179     
- Misses        98568     246485     +147917     
- Partials      19545      23604       +4059     

Flag	Coverage Δ
integration	37.5377% <25.0000%> (?)
unit	72.0648% <97.9798%> (+0.1694%)	⬆️

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

Components	Coverage Δ
dumpling	52.9656% <ø> (ø)
parser	∅ <ø> (∅)
br	52.8011% <ø> (+6.6425%)	⬆️

[YangKeao]

/retest

[YangKeao]

/retest

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-07-11 03:44:48.736652393 +0000 UTC m=+501985.971886498: ☑️ agreed by tiancaiamao.
• 2024-07-11 07:25:02.889409851 +0000 UTC m=+515200.124643964: ☑️ agreed by xhebox.

[easonn7]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: easonn7, tiancaiamao, xhebox

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• OWNERS [tiancaiamao,xhebox]
• pkg/sessionctx/variable/OWNERS [easonn7]

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[YangKeao]

/retest

[YangKeao]

/retest

[tiprow]

@YangKeao: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
fast_test_tiprow	ee804f1	link	true	/test fast_test_tiprow
tidb_parser_test	ee804f1	link	true	/test tidb_parser_test

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #54611.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #54612.