[BugFix] fix cloud native index memtable memory calculation

[luohaha]

Why I'm doing:

In previous PR #54358 , I change the memtable's memory_usage() function to:

size_t PersistentIndexMemtable::memory_usage() const {
-    return _keys_size + _map.size() * sizeof(IndexValueWithVer);
 +   return _map.bytes_used();
}

But I found that bytes_used() provide by btree map can't get the real memory usage.

What I'm doing:

Rollback to previous strategy first, will find out a better way later.

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• Interface/UI changes: syntax, type conversion, expression evaluation, display information
• Parameter changes: default values, similar parameters but with different default values
• Policy changes: use new policy to replace old one, functionality automatically enabled
• Feature removed
• Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • 3.4
  • 3.3
  • 3.2
  • 3.1
  • 3.0

[github-actions]

[Java-Extensions Incremental Coverage Report]

✅ pass : 0 / 0 (0%)

[github-actions]

[FE Incremental Coverage Report]

✅ pass : 0 / 0 (0%)

[github-actions]

[BE Incremental Coverage Report]

✅ pass : 1 / 1 (100.00%)

file detail

	path	covered_line	new_line	coverage	not_covered_line_detail
🔵	be/src/storage/lake/persistent_index_memtable.cpp	1	1	100.00%	[]

[github-actions]

@Mergifyio backport branch-3.4

[github-actions]

@Mergifyio backport branch-3.3

[mergify]

backport branch-3.4

✅ Backports have been created

• #54467 [BugFix] fix cloud native index memtable memory calculation (backport #54464) has been created for branch branch-3.4

[mergify]

backport branch-3.3

✅ Backports have been created

• #54468 [BugFix] fix cloud native index memtable memory calculation (backport #54464) has been created for branch branch-3.3

[luohaha]

https://github.com/Mergifyio backport branch-3.3.8

[mergify]

backport branch-3.3.8

✅ Backports have been created

• #54469 [BugFix] fix cloud native index memtable memory calculation (backport #54464) has been created for branch branch-3.3.8