Add OTEL export capabilities for observability and monitoring

[Jonathan-Millar]

Request: Add native OpenTelemetry (OTEL) trace export so OpenCode instances can be monitored via a standard OTEL collector/export pipeline.\n\nLinks:\n- OpenTelemetry project: https://opentelemetry.io\n- OpenTelemetry GitHub org: https://github.com/open-telemetry\n- OpenTelemetry Collector: https://github.com/open-telemetry/opentelemetry-collector\n\nWhy observability matters:\n- Lets operators diagnose latency, failures, and hot paths in production.\n- Enables service-level visibility across distributed components and clients.\n- Provides vendor-neutral telemetry that can be routed to any backend.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Feature Request: Add Langfuse Tracing #219: Feature Request: Add Langfuse Tracing
• Feature: Add sessionID to experimental.chat.system.transform hook for tracing integration #6142: Feature: Add sessionID to experimental.chat.system.transform hook for tracing integration
• [Question] detailed per-interaction telemetry (token counts and input composition) for opencode-model communications #2666: [Question] detailed per-interaction telemetry (token counts and input composition) for opencode-model communications
• Disable Telemetry #5554: Disable Telemetry
• Feature Request: Add LLM API Call Duration Logging #9037: Feature Request: Add LLM API Call Duration Logging
• [FEATURE]: Prompt + file edit logging for benchmarking #9172: [FEATURE]: Prompt + file edit logging for benchmarking
• Feature: Pass session identifier to MCP servers #6279: Feature: Pass session identifier to MCP servers

Feel free to ignore if none of these address your specific case.

[Betula-L]

I tested with Langfuse, and the missing piece seems to be handling incoming traceparent headers on the server side so request traces can attach to the upstream trace.