feat(tracing): add feature flag integration and tracking

[CalvinAllen]

Summary

Add support for tracking feature flag usage and integrating with feature flag systems. Enables A/B testing analysis and understanding which features are being used.

Use Cases

• Track which features are enabled for users
• Correlate feature flags with errors/performance
• A/B testing analysis in observability tools
• Understand feature adoption

Proposed API

// Register feature flags (typically at startup)
VsixTelemetry.RegisterFeatureFlag("dark-mode", isDarkModeEnabled);
VsixTelemetry.RegisterFeatureFlag("new-editor", () => settings.UseNewEditor);

// Track feature flag evaluation
var isEnabled = VsixTelemetry.EvaluateFeature("new-editor");
// Automatically records: feature flag name, value, and adds to current span

// Bulk registration
VsixTelemetry.RegisterFeatureFlags(new Dictionary<string, bool>
{
    ["feature-a"] = true,
    ["feature-b"] = false,
});

// Integration with feature flag providers
VsixTelemetry.SetFeatureFlagProvider(new LaunchDarklyProvider(client));

Telemetry Emitted

Spans

• feature.flag.name attribute on current span
• feature.flag.value attribute on current span

Metrics

• vs.extension.feature.evaluations (counter by flag name and value)

Resource Attributes

• All registered flags added as resource attributes for correlation

Benefits

• Correlate errors with specific feature flag combinations
• Understand feature usage patterns
• Debug issues specific to certain configurations