fix(metering): skip empty metrics and avoid emitting empty aggregates

[JmPotato]

What problem does this PR solve?

Issue Number: ref #9707.

What is changed and how does it work?

- In `ruCollector.Aggregate` and `dfsStatsCollector.Aggregate`, return `nil` instead of an empty list when no metrics are available.
- Updated `dfsStatsCollector.Collect` to ignore entries with no written bytes and no write requests, preventing unnecessary data collection.

Check List

Tests

• Unit test
• Integration test

Release note

None.

[codecov]

Codecov Report

❌ Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.83%. Comparing base (f8ab6e5) to head (b44d45c).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9786      +/-   ##
==========================================
+ Coverage   76.75%   76.83%   +0.08%     
==========================================
  Files         490      491       +1     
  Lines       78166    78466     +300     
==========================================
+ Hits        59994    60293     +299     
+ Misses      14499    14496       -3     
- Partials     3673     3677       +4     

Flag	Coverage Δ
unittests	76.83% <44.44%> (+0.08%)	⬆️

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: okJiang, rleungx

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• OWNERS [rleungx]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-09-29 02:49:18.73442397 +0000 UTC m=+52938.991155370: ☑️ agreed by rleungx.
• 2025-09-29 02:56:55.231146945 +0000 UTC m=+53395.487878345: ☑️ agreed by okJiang.