message: fmt: add public constructor to `FormattedFields`
diff: @@ -294,6 +294,16 @@ pub struct FormattedFields<E> {
  pub fields: String,
  }
  +impl<E> FormattedFields<E> {
  + /// Returns a new `FormattedFields`.
  + pub fn new(fields: String) -> Self {
  + Self {
  + fields,
  + _format_event: PhantomData,
  + }
  + }
  +}
  +
  impl<E> fmt::Debug for FormattedFields<E> {
  fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  f.debug_struct("FormattedFields")
  