Add comprehensive model tracking across all OTEL metrics and logs:
- Node execution metrics now include model_name for LLM operations
- Suggested question metrics include model_provider and model_name
- Dataset retrieval captures both embedding and rerank model info
- Updated DATA_DICTIONARY.md with complete metric label documentation
This enables granular cost tracking, performance analysis, and usage monitoring per model across all operation types.
Separate background/configuration instructions from the data dictionary:
- README.md: Overview, configuration, correlation model, content gating
- DATA_DICTIONARY.md: Pure reference format with signals and attributes
The data dictionary is now concise (465 lines vs 911) and focuses on
attribute types and relationships without verbose explanations.