Improve observability setup with the following changes:
- Replace static OnceCell with tokio::sync::OnceCell for guard management
- Add logger_level to OtelConfig for configurable tracing verbosity
- Improve telemetry initialization with better error handling
- Enhance logging filters and span configuration
Breaking Changes:
- Configuration now requires logger_level field
- Global guard management uses async-safe primitives
Example config:
observability:
endpoint: "http://localhost:4317"
logger_level: "debug" # New required field
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