futures: Make WithSubscriber work with existing Dispatch

The upstream change that made `Dispatch` no longer implement `Subscriber`
broke this a bit, since a `Dispatch` can no longer be passed to
`with_subscriber`. To make this also work with unwrapped subscribers,
we'll probably want an impl of `From<Subscriber>` for `Dispatch`.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tokio-trace-futures/src/lib.rs

@@ -21,11 +21,11 @@ pub trait Instrument: Sized {
 pub trait WithSubscriber: Sized {
  fn with_subscriber<S>(self, subscriber: S) -> WithDispatch<Self>
  where
- S: Subscriber + Send + Sync + 'static,
+ S: Into<Dispatch>,
  {
  WithDispatch {
  inner: self,
- dispatch: Dispatch::new(subscriber),
+ dispatch: subscriber.into(),
  }
  }
 }