[DNM][misc](bvar) expose file uploading thread pool queue size

[freemandealer]

What problem does this PR solve?

expose thread pool queue size for better monitoring upload pressure

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • [x ] No need to test or manual test. Explain why:
    • [x ] This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • [x ] No. Just internal info exposure for debug reason
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[freemandealer]

run buildall

[doris-robot]

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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[github-actions]

clang-tidy made some suggestions

[github-actions]

warning: 'auto pool' can be declared as 'auto *pool' [readability-qualified-auto]

Suggested change

	auto pool = ExecEnv::GetInstance()->s3_file_upload_thread_pool();
	auto *pool = ExecEnv::GetInstance()->s3_file_upload_thread_pool();

[doris-robot]

TeamCity be ut coverage result:
Function Coverage: 37.75% (9815/26003)
Line Coverage: 29.05% (82126/282682)
Region Coverage: 28.16% (42217/149898)
Branch Coverage: 24.74% (21393/86470)
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