external: set the period of checking merge iter hotspot according to the key size

[wjhuang2016]

What problem does this PR solve?

Issue Number: close #47923

Problem Summary:
If a hotspot is detected, the reader would change to concurrent mode and read 32M data by default.
If the hotspot shifts to other files, the previous reader will drop the unread byte to release memory. If dropping too many bytes, the whole performance would be very bad.

What is changed and how it works?

Set the period of checking merge iter hotspot according to the key size.
For example, if the average key size is 0.1K (by sampling), then we can check the hotspot every
(32M / 0.1K) = 320000 KVs.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  before:

after:

• 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 #48063 (a803576) into master (3682bd8) will increase coverage by 1.1475%.
Report is 14 commits behind head on master.
The diff coverage is 100.0000%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #48063        +/-   ##
================================================
+ Coverage   71.6145%   72.7621%   +1.1475%     
================================================
  Files          1401       1424        +23     
  Lines        405923     413847      +7924     
================================================
+ Hits         290700     301124     +10424     
+ Misses        95448      93774      -1674     
+ Partials      19775      18949       -826     

Flag	Coverage Δ
integration	42.6365% <0.0000%> (?)
unit	71.5895% <100.0000%> (-0.0250%)	⬇️

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

Components	Coverage Δ
dumpling	54.0503% <ø> (ø)
parser	∅ <ø> (∅)
br	48.6316% <100.0000%> (-4.2892%)	⬇️

[lance6716]

it's a bit safer to check sampleKeySize/sampleKeyCnt is zero

[wjhuang2016]

/retest

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lance6716, tangenta

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• br/pkg/lightning/OWNERS [lance6716]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-10-30 03:46:37.622695318 +0000 UTC m=+2838395.209805463: ☑️ agreed by lance6716.
• 2023-10-30 15:59:57.228386382 +0000 UTC m=+2882394.815496527: ☑️ agreed by tangenta.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #48121.