statistics: fix unnecessary index fmsketch loading (#42074)

[ti-chi-bot]

This is an automated cherry-pick of #42074

What problem does this PR solve?

Issue Number: close #42052

Problem Summary:

What is changed and how it works?

When loading index stats from storage, we don't need to load index fmsketch unless loadAll is true. We only use index fmsketch when merging partition stats to global stats, where loadAll is true.

Check List

Tests

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

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

[ti-chi-bot]

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[ti-chi-bot]

PR needs rebase.

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