feat(observability): Implement OTel telemetry parity for agents and tools

[Starefossen]

Issue for this PR

Supersedes #5245

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

(Note: This PR supersedes #5245. While OTel was previously implemented in the codebase, granular agent and tool metrics were still missing.)

OpenCode lacked granular observability into agent actions and tool usage compared to other popular AI clients. Without standardized metrics for these events, it was difficult to debug failing agent interactions or build robust dashboards for tool success rates.

This PR implements telemetry parity by bringing the agent and tool instrumentation in line with the official OpenTelemetry Generative AI Semantic Conventions (gen_ai.*).

Specifically, this implements:

• Resource Context: Configured the OpenTelemetry SDK to attach standard device, session, and environment identifiers to all emitted metrics.
• Agent Loop Instrumentation: Instrumented SessionProcessor to emit the agentTurnCount metric. It aligns with the OTel spec by capturing logical gen_ai.operation.name="invoke_agent" turns while correctly ignoring internal context-window compactions to prevent metric inflation.
• Tool Execution Tracking: Wrapped tool resolutions in tools.ts to capture toolCallCountTotal and toolCallDurationSeconds. This accurately tracks execution times and leverages standard OTel attributes like error.type and success states, providing granular visibility into infrastructure vs. logical tool failures.

How did you verify your code works?

• Verified that all TypeScript typings and linter rules pass locally.
• Conducted an adversarial code review to ensure edge cases (like network retries and context compaction) do not double-count metrics.
• Passed full mise check pipeline.
• Verified that the generated attributes align with standard OpenTelemetry GenAI semantics.

Screenshots / recordings

N/A (Backend / Observability change)

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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Implemented with the help of Antigravity CLI (Gemini 3.1 Pro High)

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

I found one potentially related PR that should be reviewed:

Related PR:

• feat: integrate OpenTelemetry #5245 - feat: integrate OpenTelemetry

Why it's related: This PR also covers OpenTelemetry integration. It may be addressing similar observability goals or could be an earlier approach to the same feature. You should verify whether #5245 is already merged, stale, or covers different scope than the agent/tool instrumentation in PR #34633.

[github-actions]

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