OpenTelemetry observer implementation

[JaimeStill]

Migrated from tailored-agentic-units/tau-orchestrate#4

Summary

Provide an Observer implementation that exports events as OpenTelemetry spans and metrics, enabling integration with standard observability platforms (Jaeger, Grafana, Datadog).

Requirements

• Map workflow execution to OTel spans (graph → parent span, nodes → child spans)
• Map EventType to span events/attributes
• Export metrics (counters, histograms) for execution statistics
• Context propagation compatible with OTel trace context
• Optional dependency (separate package to avoid mandatory OTel dependency)

Design Constraints

• No core package coupling: consider orchestrate/otel to avoid adding OTel as a transitive dependency for the entire module
• Registry compatible: must integrate with the existing string-based registry pattern
• Backward compatible: existing Observer interface must not change

Acceptance Criteria

• OpenTelemetry observer produces valid spans and metrics consumable by standard OTel backends
• All new observer implementations pass through the existing registry pattern
• No performance regression for existing NoOpObserver and SlogObserver usage
• Extensibility demonstrated through at least one custom observer implementation