Create encoder for parametrized case class using Shapeless HList

Question

I want to implement ItemEncoder type class for Element class and encode elements collection

trait Tag
case class Element[A](attrName: String, value: A) extends Tag

val elements = Element("name", "foo") :: Element("age", 37) :: Element("married", true) :: HNil

trait ItemEncoder[A] {
  // labeled generic - field name and field value
  def encode(element: Element[A]): List[Item]
}

object ItemEncoder {
  def apply[A](implicit encoder: ItemEncoder[A]): ItemEncoder[A] = encoder

  def instance[A](f: Element[A] => List[Item]): ItemEncoder[A] =
    new ItemEncoder[A] {
      override def encode(element: Element[A]) = f(element)
    }
}

implicit val stringEncoder: ItemEncoder[String] =
  ItemEncoder.instance(element => {
    val item = new Item()
    item.setString(element.attrName, element.value)
    List(item)
  })

implicit val intEncoder: ItemEncoder[Int] =
  ItemEncoder.instance(element => {
    val item = new Item()
    item.setInt(element.attrName, element.value)
    List(item)
  })

implicit val booleanEncoder: ItemEncoder[Boolean] =
  ItemEncoder.instance(element => {
    val item = new Item()
    item.setBoolean(element.attrName, element.value)
    List(item)
  })

implicit def hlistEncoder[K, H, T <: HList](
  implicit
  hEncoder: ItemEncoder[H],
  tEncoder: ItemEncoder[T]
): ItemEncoder[Element[K] :: T] = {
  ItemEncoder.instance {
    ???
  }
}