*: add variable to control async merge global stats

[hawkingrei]

What problem does this PR solve?

Issue Number: ref #47275

Problem Summary:

What is changed and how it works?

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.

None

[codecov]

Codecov Report

Merging #47806 (77751fe) into master (f4a139f) will increase coverage by 0.9301%.
Report is 9 commits behind head on master.
The diff coverage is 48.8235%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #47806        +/-   ##
================================================
+ Coverage   71.8610%   72.7911%   +0.9301%     
================================================
  Files          1398       1422        +24     
  Lines        405256     411862      +6606     
================================================
+ Hits         291221     299799      +8578     
+ Misses        94397      93196      -1201     
+ Partials      19638      18867       -771     

Flag	Coverage Δ
integration	41.4482% <2.3529%> (?)
unit	71.8557% <48.8235%> (-0.0053%)	⬇️

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

Components	Coverage Δ
dumpling	53.9913% <ø> (ø)
parser	∅ <ø> (∅)
br	48.7187% <ø> (-4.2488%)	⬇️

[winoros]

we may need to add a bootstrap upgrade func to make the upgraded cluster keep the old behavior

[easonn7]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: easonn7, qw4990, winoros

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Needs approval from an approver in each of these files:

• OWNERS [qw4990,winoros]
• pkg/session/OWNERS [easonn7]
• pkg/sessionctx/variable/OWNERS [easonn7]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-10-20 07:01:44.343658023 +0000 UTC m=+1986101.930768164: ☑️ agreed by qw4990.
• 2023-10-20 08:21:20.066312639 +0000 UTC m=+1990877.653422780: ☑️ agreed by winoros.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #47880.