[FEATURE] Add subagent_type to OTEL telemetry for Task tool calls

[tonybentley]

Summary

When using the Task tool to spawn subagents (e.g., Explore, claude-code-guide, Plan, or custom agents), the OTEL telemetry does not capture which agent type was invoked. This makes it impossible to build observability dashboards showing agent usage patterns.

Current Behavior

For tool_result events where tool_name="Task":

• tool_parameters is null (unlike Bash which logs command details)
• No subagent_type field exists in structured metadata

Example current telemetry for Task:

{
  "tool_name": "Task",
  "tool_parameters": null,
  "session_id": "...",
  "success": "true",
  "duration_ms": "8801"
}

Expected Behavior

Task tool telemetry should include the subagent_type parameter:

{
  "tool_name": "Task",
  "subagent_type": "Explore",
  "tool_parameters": null,
  "session_id": "...",
  "success": "true",
  "duration_ms": "8801"
}

Use Case

Organizations monitoring Claude Code usage want to understand:

• Which subagents are most frequently used (Explore, Plan, custom agents)
• Agent usage distribution per session
• Cost/time attribution by agent type
• Custom vs system agent utilization

Environment

• Claude Code version: 2.0.74
• OTEL exporter: OTLP gRPC to local collector
• Observability stack: OTEL Collector → Prometheus/Loki → Grafana

Additional Context

This gap was discovered while building a local observability POC. The telemetry correctly captures tool_name for all tools, but the subagent_type parameter passed to the Task tool is not emitted as a structured metadata field.