Update tracing example with working distributed trace

Replaced old trace example with verified working trace that shows complete
distributed tracing across all three services.

Changes:
- Updated curl command and response with actual working output
- New trace ID: 4be1151c0bdcd5625ac7e02b98d95bd5 (old: 4e8d5a25ae6f8f8d737b46625920fbb9)
- Added kubectl commands to search and fetch traces from Tempo API
- Documented complete trace structure with 8 spans across 3 services:
  * Frontend: 3 spans (GET /api/process, frontend-process, POST) - 221ms
  * Middleware: 3 spans (POST /api/transform, middleware-transform, GET) - 186ms
  * Backend: 2 spans (GET /api/data, backend-get-data) - 104ms
- Added detailed span annotations explaining each span's role
- Included timing information showing distributed request flow
- Documented W3C Trace Context header propagation

This trace was generated after fixing health check noise by excluding /health
endpoints from instrumentation, which allows API traces to be properly exported
and visible in Tempo.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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