message: Fix spans failing to record fmt values
  Looks like I inadvertantly broke this in
  with an accidental `!` in
  `record_value_fmt`. Oops!
diff: @@ -634,7 +634,7 @@ impl<'a> Inner<'a> {
  /// Record a precompiled set of format arguments value.
  pub(crate) fn record_value_fmt(&self, field: &Key, value: fmt::Arguments) {
  - if !self.meta.contains_key(field) {
  + if self.meta.contains_key(field) {
  self.subscriber.record_fmt(&self.id, field, value)
  }
  }
  