feat(metrics): add health check status metric

[CalvinAllen]

Summary

Expose extension health status as a metric that can be monitored in dashboards. Allows operators to quickly see if extensions are healthy across a fleet of VS installations.

Proposed API

// Report health status
VsixTelemetry.ReportHealth(HealthStatus.Healthy);
VsixTelemetry.ReportHealth(HealthStatus.Degraded, "Cache connection slow");
VsixTelemetry.ReportHealth(HealthStatus.Unhealthy, "Database unreachable");

// Or with a health check function
VsixTelemetry.RegisterHealthCheck("database", () => 
{
    return database.IsConnected 
        ? HealthCheckResult.Healthy() 
        : HealthCheckResult.Unhealthy("Connection lost");
});

// Periodic health reporting (background)
VsixTelemetry.EnablePeriodicHealthReporting(interval: TimeSpan.FromMinutes(1));

Metrics Emitted

• vs.extension.health.status (gauge: 0=unhealthy, 1=degraded, 2=healthy)
• vs.extension.health.check.duration (histogram per check)
• vs.extension.health.check.failures (counter)

Benefits

• Quick visibility into extension health
• Enables alerting on degraded status
• Useful for monitoring dashboards
• Follows ASP.NET Core health check patterns