code: func WithTrace(ctx context.Context, traceID, spanID, parentID string) context.Context {
  	if parentID != "" {
  		ctx = context.WithValue(ctx, parentSpanKey, parentID)
  	}
  	ctx = context.WithValue(ctx, traceKey, traceID)
  	ctx = context.WithValue(ctx, spanKey, spanID)
  	return ctx
  }
docs: // WithTrace returns a context containing the given trace, span and parent span
  // IDs.