message: 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`.
diff: @@ -5,7 +5,7 @@ extern crate tokio;
  extern crate tokio_trace;
  use futures::{Async, Future, Poll, Sink, StartSend, Stream};
  -use tokio_trace::{dispatcher, Dispatch, Span, Subscriber};
  +use tokio_trace::{dispatcher, Dispatch, Span};
  pub mod executor;
  @@ -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(),
  }
  }
  }
  