ci: add PR size labeler workflow

[cijothomas]

Adds a GitHub Actions workflow that automatically labels pull requests by size (size/XS through size/XL) based on changed line count, and posts a friendly reminder comment when a PR hits size/XL (500+ lines).

Uses CodelyTV/pr-size-labeler (390+ stars, MIT, actively maintained), pinned to a commit SHA per OSSF Scorecard best practices.

Thresholds (Kubernetes / Prow-aligned)

Label	Lines changed
size/XS	0–9
size/S	10–29
size/M	30–99
size/L	100–499
size/XL	500+ (triggers reminder comment)

Notes

• Triggered via pull_request_target (same pattern as the existing labeler.yml). No PR code checkout occurs, so this is safe with fork PRs.
• Generated/vendored files (go.sum, Cargo.lock, *.pb.go, testdata/, vendor/) are ignored to avoid skewing counts.
• Draft PRs are skipped.
• Does not fail CI — only labels and (for XL) comments.

Follow-up (if approved)

The labels (size/XS ... size/XL) will need to be created in the repo (one-time admin task). Suggested color scheme matches Kubernetes' Prow defaults.

Follow-up to #2864 (comment) from @drewrelmas.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.92%. Comparing base (672d665) to head (c774142).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2989      +/-   ##
==========================================
- Coverage   85.92%   85.92%   -0.01%     
==========================================
  Files         725      725              
  Lines      275605   275605              
==========================================
- Hits       236811   236806       -5     
- Misses      38270    38275       +5     
  Partials      524      524              

Components	Coverage Δ
otap-dataflow	87.04% <ø> (-0.01%)	⬇️
query_abstraction	80.61% <ø> (ø)
query_engine	89.57% <ø> (ø)
otel-arrow-go	52.45% <ø> (ø)
quiver	92.25% <ø> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.