metrics: update distsql_copr_resp_size unit

[zimulala]

What problem does this PR solve?

Issue Number: ref #59990

Problem Summary:
In TiDB clusters with numerous nodes and high request loads, tidb_distsql_copr_resp_size is monitoring a large amount of data, consider reducing the number of buckets to reduce its data size.

What changed and how does it work?

Change the minimum unit from B to KiB. This can reduce the number of buckets by 50%

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  Before this PR:

After this PR:

• 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

[tiprow]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.2193%. Comparing base (b72ff3b) to head (5188ebf).
Report is 22 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #59991        +/-   ##
================================================
+ Coverage   73.0771%   75.2193%   +2.1421%     
================================================
  Files          1702       1752        +50     
  Lines        470092     481807     +11715     
================================================
+ Hits         343530     362412     +18882     
+ Misses       105392      96952      -8440     
- Partials      21170      22443      +1273     

Flag	Coverage Δ
integration	49.0867% <100.0000%> (?)
unit	72.7328% <100.0000%> (+0.4503%)	⬆️

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Components	Coverage Δ
dumpling	52.6910% <ø> (ø)
parser	∅ <ø> (∅)
br	62.3996% <ø> (+15.1769%)	⬆️

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-03-12 06:19:15.366465074 +0000 UTC m=+336710.971994000: ☑️ agreed by lance6716.
• 2025-03-12 06:47:00.647922557 +0000 UTC m=+338376.253451484: ☑️ agreed by hawkingrei.