message: serde: implement `AsSerde` for `FieldSet`
  ## Motivation
  I've wanted to serialize fieldset of current span.
  ## Solution
  Expose already existing `SerializeFieldSet` for users by implementing `AsSerde` for `FieldSet`.
diff: @@ -533,6 +533,14 @@ impl<'a> AsSerde<'a> for Level {
  }
  }
  +impl<'a> AsSerde<'a> for FieldSet {
  + type Serializable = SerializeFieldSet<'a>;
  +
  + fn as_serde(&'a self) -> Self::Serializable {
  + SerializeFieldSet(self)
  + }
  +}
  +
  impl<'a> self::sealed::Sealed for Event<'a> {}
  impl<'a> self::sealed::Sealed for Attributes<'a> {}
  @@ -545,6 +553,8 @@ impl<'a> self::sealed::Sealed for Record<'a> {}
  impl<'a> self::sealed::Sealed for Metadata<'a> {}
  +impl self::sealed::Sealed for FieldSet {}
  +
  mod sealed {
  pub trait Sealed {}
  }
  