lightning: fix an incorrect metric

[wjhuang2016]

What problem does this PR solve?

Issue Number: close #51947

Problem Summary:

What changed and how does it work?

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 #52111 (2c0202f) into master (311eef9) will decrease coverage by 15.9422%.
Report is 10 commits behind head on master.
The diff coverage is 66.6666%.

Additional details and impacted files

@@                Coverage Diff                @@
##             master     #52111         +/-   ##
=================================================
- Coverage   70.6704%   54.7283%   -15.9422%     
=================================================
  Files          1486       1599        +113     
  Lines        439434     610726     +171292     
=================================================
+ Hits         310550     334240      +23690     
- Misses       109365     253275     +143910     
- Partials      19519      23211       +3692     

Flag	Coverage Δ
integration	36.7313% <66.6666%> (?)

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

Components	Coverage Δ
dumpling	53.9957% <ø> (ø)
parser	∅ <ø> (∅)
br	51.6156% <66.6666%> (+5.7963%)	⬆️

[lance6716]

better to move to ⬇️

tidb/br/pkg/lightning/backend/external/engine.go

Lines 326 to 329 in bd17acd

	// release the reference of e.memKVsAndBuffers
	e.memKVsAndBuffers.keys = nil
	e.memKVsAndBuffers.values = nil
	e.memKVsAndBuffers.memKVBuffers = nil

resetting these fields in one place

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-03-27 07:51:45.117331701 +0000 UTC m=+2054332.139578089: ☑️ agreed by ywqzzy.
• 2024-03-27 07:56:14.436116643 +0000 UTC m=+2054601.458363031: ☑️ agreed by lance6716.