message: add with_subscriber to span
  Intended to allow library authors to access a span's dispatcher for
  out-of-band interaction with subscribers/layers.
  see for details
  Closes
diff: @@ -821,6 +821,17 @@ impl Span {
  }
  }
  }
  +
  + /// Invokes a function with a reference to this span's ID and subscriber.
  + ///
  + /// if this span is enabled, the provided function is called, and the result is returned.
  + /// If the span is disabled, the function is not called, and this method returns `None`
  + /// instead.
  + pub fn with_subscriber<T>(&self, f: impl FnOnce((&Id, &Dispatch)) -> T) -> Option<T> {
  + self.inner
  + .as_ref()
  + .map(|inner| f((&inner.id, &inner.subscriber)))
  + }
  }
  impl cmp::PartialEq for Span {
  