*: support setting txn-entry-size-limit by sys var

[zyguan]

What problem does this PR solve?

Issue Number: close #49236

Problem Summary: allow setting txn-entry-size-limit via system variable.

What changed and how does it work?

Add tidb_txn_entry_size_limit to address the issue.

• Scope: GLOBAL | SESSION
• Persists to cluster: Yes
• Applies to hint SET_VAR: No
• Data type: Integer
• Default value: 0 ( disabled, use config by default )
• Range: 0 to 125829120 (120MiB)
• Units: byte
• This variable is used to overwrite txn-entry-size-limit. When setting it in global scope, it also affects internal sessions used by DDL backfilling.

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.

Support setting txn-entry-size-limit by system variable.

[tiprow]

Hi @zyguan. Thanks for your PR.

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[codecov]

Codecov Report

Merging #49237 (549f7f9) into master (ecb4783) will increase coverage by 0.6541%.
Report is 15 commits behind head on master.
The diff coverage is 51.5151%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #49237        +/-   ##
================================================
+ Coverage   70.9581%   71.6122%   +0.6541%     
================================================
  Files          1368       1428        +60     
  Lines        397092     435191     +38099     
================================================
+ Hits         281769     311650     +29881     
- Misses        95625     104315      +8690     
+ Partials      19698      19226       -472     

Flag	Coverage Δ
integration	43.9268% <48.3871%> (?)
unit	71.0683% <100.0000%> (+0.1102%)	⬆️

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

Components	Coverage Δ
dumpling	53.9663% <ø> (ø)
parser	∅ <ø> (∅)
br	47.2043% <ø> (-5.6998%)	⬇️

[cfzjywxk]

/cc
@easonn7 @benmaoer PTAL

[cfzjywxk]

LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-12-22 11:21:53.880818005 +0000 UTC m=+1219204.918044933: ☑️ agreed by cfzjywxk.
• 2023-12-25 06:35:23.079516588 +0000 UTC m=+1461214.116743514: ☑️ agreed by crazycs520.

[easonn7]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cfzjywxk, crazycs520, easonn7, tangenta

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Needs approval from an approver in each of these files:

• OWNERS [cfzjywxk,crazycs520,tangenta]
• pkg/ddl/OWNERS [tangenta]
• pkg/sessionctx/variable/OWNERS [easonn7]

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