message: Fix span creation with no fields requiring a trailing comma
diff: @@ -114,6 +114,7 @@ macro_rules! static_meta {
  #[macro_export]
  macro_rules! span {
  + ($name:expr) => { span!($name,) };
  ($name:expr, $($k:ident = $val:expr),*) => {
  $crate::Span::new(
  Some($name),
  