[CLEAN] Synthetic Benchmark PR #28582 - add service layer OTel Span

[tomerqodo]

Benchmark PR langgenius#28582

Type: Clean (correct implementation)

Original PR Title: add service layer OTel Span
Original PR Description: > [!IMPORTANT]

1. Make sure you have read our contribution guidelines
2. Ensure there is an associated issue and you have been assigned to it
3. Use the correct syntax to link this PR: Fixes #<issue number>.

Summary

part of langgenius#28446

OTel service-layer instrumentation implemented using decorators

Screenshots

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

Original PR URL: langgenius#28582