Add OpenTelemetry observability support

[pcomans]

Summary

Add OpenTelemetry-based observability to RubyLLM, enabling users to send traces to any compatible backend (LangSmith, Honeycomb, New Relic, etc.).

Follows the OpenTelemetry Semantic Conventions for GenAI.

What's Included

• Chat completion tracing: chat {model_id} spans (e.g., chat gpt-4o) with model, provider, token counts, and GenAI semantic convention attributes
• Streaming support: Single span wraps entire streaming operation; token counts and content recorded on completion
• Tool call tracing: execute_tool {tool_name} child spans (e.g., execute_tool get_weather) with tool name, arguments, and results
• Session tracking: UUID per Chat instance for grouping multi-turn conversations. ActiveRecord chats use namespaced record IDs (ClassName:id) to prevent collisions across models
• Custom metadata: Attach arbitrary metadata via chat.with_metadata(user_id: 123) for filtering/debugging. Native types (int, float, bool) preserved for better backend filtering
• Content logging: Opt-in prompt/completion logging with configurable truncation
• LangSmith compatibility: Optional tracing_langsmith_compat mode adds LangSmith-specific attributes (langsmith.span.kind, input.value/output.value panels)
• Config scoping: Tracing respects per-context configuration when using RubyLLM.context
• Fail-fast validation: Raises ConfigurationError with helpful message if tracing enabled but OpenTelemetry gems not installed
• NullTracer pattern: Minimal overhead when tracing disabled

Configuration

RubyLLM.configure do |config|
  config.tracing_enabled = true                  # Explicit opt-in required
  config.tracing_log_content = true              # Log prompts/completions (default: false)
  config.tracing_max_content_length = 10_000     # Truncation limit (default: 10000)
  config.tracing_langsmith_compat = true         # Add LangSmith-specific attributes (auto-sets metadata prefix)
end

Not in Scope

Intentionally deferred to keep this PR focused:

• Other operations (embed, paint, transcribe, moderate): Lower priority; can add incrementally based on demand

[pcomans]

@crmne I'd love to get this merged. Let me know how I can support you.