coprocessor: avoid unnecessary encode when collect_column_stats

[xuyifangreeneyes]

What is changed and how it works?

Issue Number: ref #14231

What's Changed:

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression
  • Consumes more CPU
  • Consumes more MEM
• Breaking backward compatibility

Release note

None

[ti-chi-bot]

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[xuyifangreeneyes]

/run-build

[xuyifangreeneyes]

/run-build

[xuyifangreeneyes]

/run-build

[ti-chi-bot]

PR needs rebase.

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