message: fix lifetime issues with event macro
diff: @@ -130,14 +130,15 @@ macro_rules! span {
  macro_rules! event {
  (target: $target:expr, $lvl:expr, { $($k:ident = $val:expr),* }, $($arg:tt)+ ) => ({
  { let field_values: &[& dyn $crate::Value] = &[ $( & $val),* ];
  - $crate::Event {
  + use $crate::Subscriber;
  + $crate::Dispatcher::current().observe_event(&$crate::Event {
  timestamp: ::std::time::Instant::now(),
  parent: $crate::Span::current(),
  follows_from: &[],
  meta: &static_meta!(@ $target, $lvl, $($k),* ),
  field_values: &field_values[..],
  message: format_args!( $($arg)+ ),
  - };
  + });
  }
  });
  @@ -259,12 +260,6 @@ impl<'event, 'meta: 'event> Event<'event, 'meta> {
  }
  }
  -impl<'event, 'meta> Drop for Event<'event, 'meta> {
  - fn drop(&mut self) {
  - Dispatcher::current().observe_event(self);
  - }
  -}
  -
  // ===== impl Span =====
  impl Span {
  