Add OpenTelemetry instrumentation infrastructure for observability

[Copilot]

Adds OpenTelemetry-based observability infrastructure for VS Code Copilot Chat agent, modeled after Gemini CLI's telemetry system.

Changes

Core Service Interface

• IOpenTelemetryService with typed attributes for tool calls, API requests, agent lifecycle, file operations
• NullOpenTelemetryService for zero-overhead disabled state
• FileOpenTelemetryService writes JSON lines to configurable output file

Configuration

New settings under github.copilot.chat.openTelemetry.*:

• enabled, target, otlpEndpoint, otlpProtocol, outfile, logPrompts

Privacy-first: logPrompts: false (default) excludes user prompts from telemetry.

Event Types

Logs:    copilot_chat.{config,user_prompt,tool_call,api_request,api_response,agent.start,agent.finish}
Metrics: copilot_chat.{session.count,tool.call.count,tool.call.latency,token.usage,agent.duration}
Spans:   Session, API request, and tool invocation with parent-child relationships

Usage

{
  "github.copilot.chat.openTelemetry.enabled": true,
  "github.copilot.chat.openTelemetry.outfile": "~/.copilot/telemetry.log"
}

Output format (JSON lines):

{"timestamp":"...","type":"log","name":"copilot_chat.tool_call","attributes":{"function_name":"read_file","success":true,"tool_type":"native"},"common":{"session.id":"...","installation.id":"..."}}

Files

• src/platform/telemetry/common/openTelemetryService.ts - Interface + types
• src/platform/telemetry/common/nullOpenTelemetryService.ts - Disabled impl
• src/platform/telemetry/node/fileOpenTelemetryService.ts - File output impl
• src/platform/telemetry/vscode-node/openTelemetryServiceImpl.ts - Config-aware wrapper
• Service registered in src/extension/extension/vscode-node/services.ts

Not Included

• Actual instrumentation callsites (tool loop, API handlers, agent lifecycle)
• OTLP HTTP/gRPC exporters (only file output implemented)

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