message: futures: Instrumented#{inner,inner_mut}
  ## Motivation
  Currently it is not possible to implement 3rd party traits for
  `Instrumented` because there is no way to access the `inner` type
  without consuming the `Instrumented` wrapper.
  ## Solution
  This adds `inner` and `inner_mut` methods to `Instrumented`
diff: @@ -277,6 +277,16 @@ impl<T> Instrumented<T> {
  &mut self.span
  }
  + /// Borrows the wrapped type.
  + pub fn inner(&self) -> &T {
  + &self.inner
  + }
  +
  + /// Mutably borrows the wrapped type.
  + pub fn inner_mut(&mut self) -> &mut T {
  + &mut self.inner
  + }
  +
  /// Consumes the `Instrumented`, returning the wrapped type.
  ///
  /// Note that this drops the span.
  