[feat](txn) Add txn write amplification brpc metrics for sub txn load

[wyxxxcat]

What problem does this PR solve?

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)
  • No need to test or manual test. Explain why:
    • 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?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

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

[hello-stephen]

Thank you for your contribution to Apache Doris.
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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?

[wyxxxcat]

run buildall

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.

[gavinchou]

/review

[github-actions]

Review result: no blocking issues found. The added stats recording in commit_txn_with_sub_txn is consistent with the existing commit_txn_immediately and commit_txn_eventually call sites and occurs after pending lazy commits are advanced, before the commit mutation is built.

Critical checkpoint conclusions:

• Goal and proof: Goal appears to be adding txn commit count metrics for sub-txn load; the code accomplishes this by invoking the existing recorder on the sub-txn path. No new test is included; acceptable for this observability-only addition, but CI/build coverage should still compile the cloud target.
• Scope: Change is small and focused.
• Concurrency/lifecycle: No new shared state or lifecycle management is introduced beyond existing bvar recorders.
• Config/compatibility/protocol: No new config, storage format, RPC schema, or compatibility behavior.
• Parallel paths: The sub-txn path now matches the existing immediate/eventual commit paths.
• Error handling/data correctness/MoW: No commit control flow, transaction state, version visibility, or MoW metadata logic is changed.
• Testing: No PR test evidence provided; risk is limited to compile/runtime observability because this is a metrics-only call.
• Observability: Improves coverage for sub-txn commit metrics.
• User focus: No additional user-provided review focus.