xaml designer extension: how to access attached properties

[assembly: ProvideMetadata(typeof(TestControls.DesignTools.Metadata))]
namespace TestControls.DesignTools
{
    internal class Metadata : IProvideAttributeTable
    {
        public AttributeTable AttributeTable
        {
            get
            {
                AttributeTableBuilder builder = new AttributeTableBuilder();

                builder.AddCustomAttributes("TestControls.CustomControl1", new FeatureAttribute(typeof(AdornerProvider)));

                return builder.CreateTable();
            }
        }
    }
}