statistics: update some vars for internal sessions correctly

[Rustin170506]

What problem does this PR solve?

Issue Number: close #53972

Problem Summary:

What changed and how does it work?

We need to always update these vars for internal sessions. Otherwise, it won't work. And we will use the wrong default value.

See more at the issue.

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.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

修复 tidb_enable_async_merge_global_stats 和 tidb_analyze_partition_concurrency 在自动分析无法生效的问题
Fix tidb_enable_async_merge_global_stats and tidb_analyze_partition_concurrency not working in auto-analysis

[codecov]

Codecov Report

Attention: Patch coverage is 65.38462% with 9 lines in your changes missing coverage. Please review.

Project coverage is 55.0690%. Comparing base (7a18952) to head (ec24144).
Report is 15 commits behind head on master.

Additional details and impacted files

@@                Coverage Diff                @@
##             master     #53977         +/-   ##
=================================================
- Coverage   72.5434%   55.0690%   -17.4745%     
=================================================
  Files          1508       1627        +119     
  Lines        431744     604879     +173135     
=================================================
+ Hits         313202     333101      +19899     
- Misses        99141     249554     +150413     
- Partials      19401      22224       +2823     

Flag	Coverage Δ
integration	35.5112% <65.3846%> (?)
unit	71.4346% <50.0000%> (-0.0950%)	⬇️

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	42.5351% <ø> (+0.4020%)	⬆️

[fixdb]

+1

[Rustin170506]

/hold

[Rustin170506]

🔢 Self-check (PR reviewed by myself and ready for feedback.)

[Rustin170506]

/unhold

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: fixdb, hawkingrei

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• pkg/statistics/OWNERS [fixdb,hawkingrei]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-06-13 02:32:13.463038251 +0000 UTC m=+604687.516350175: ☑️ agreed by fixdb.
• 2024-06-13 04:02:07.752488975 +0000 UTC m=+610081.805800897: ☑️ agreed by hawkingrei.

[Rustin170506]

/hold

[Rustin170506]

/unhold

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #53986.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #53987.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #53988.