message: trace: Allow `Span` IDs to be converted back to `u64`s
  ## Motivation
  As described in subscribers have no way to get the numeric value of a span ID back _out_ of a `Span`.
  ## Solution
  Add a `Span::into_u64` method that returns the inner `u64` span ID
  Closes
diff: /// Identifies a span within the context of a process.
  ///
  -/// Span IDs are used primarily to determine of two handles refer to the same
  +/// Span IDs are used primarily to determine if two handles refer to the same
  /// span, without requiring the comparison of the span's fields.
  ///
  /// They are generated by [`Subscriber`](::Subscriber)s for each span as it is
  @@ -19,4 +19,9 @@ impl Span {
  pub fn from_u64(u: u64) -> Self {
  Span(u)
  }
  +
  + /// Returns the span's ID as a `u64`.
  + pub fn into_u64(&self) -> u64 {
  + self.0
  + }
  }
  