[metrics](warmup) add bvar for skipped warmup rowsets

[kaijchen]

What problem does this PR solve?

Problem Summary:

Adds the bvar file_cache_event_driven_warm_up_skipped_rowset_num to the source cluster Backend to improve observability of the cache warmup process.

During event-driven warmup, rowsets can be skipped if the Backend fails to find the tablet's replica location information from a TGetTabletReplicaInfosRequest.

This new metric makes it possible to monitor and alert on these events, which helps in diagnosing incomplete cache warmups.

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?

[kaijchen]

run buildall

[hello-stephen]

BE UT Coverage Report

Increment line coverage 0.00% (0/1) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	52.40% (17626/33636)
Line Coverage	37.62% (160008/425324)
Region Coverage	32.14% (121865/379128)
Branch Coverage	33.50% (53430/159504)

[liaoxin01]

LGTM

[github-actions]

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

[github-actions]

PR approved by anyone and no changes requested.

[dataroaring]

LGTM