fixup! fixup! fixup! fixup! fixup! Add mapK to Tracer, `SpanBuild…

…er`, etc.

remove unnecessary Span.MappedK

core/trace/src/main/scala/org/typelevel/otel4s/trace/Span.scala

@@ -97,7 +97,7 @@ trait Span[F[_]] extends SpanMacro[F] {
     backend.end(timestamp)
 
   /** Modify the context `F` using the transformation `f`. */
-  def mapK[G[_]](f: F ~> G): Span[G] = new Span.MappedK(this)(f)
+  def mapK[G[_]](f: F ~> G): Span[G] = Span.fromBackend(backend.mapK(f))
 
   /** Modify the context `F` using an implicit [[KindTransformer]] from `F` to
     * `G`.
@@ -206,9 +206,4 @@ object Span {
     new Span[F] {
       def backend: Backend[F] = back
     }
-
-  /** Implementation for [[Span.mapK]]. */
-  private class MappedK[F[_], G[_]](span: Span[F])(f: F ~> G) extends Span[G] {
-    def backend: Backend[G] = span.backend.mapK(f)
-  }
 }