message: Add on_follows_from to OpenTelemetryLayer
diff: @@ -415,6 +415,30 @@ where
  }
  }
  + fn on_follows_from(&self, id: &Id, follows: &Id, ctx: Context<S>) {
  + let span = ctx.span(id).expect("Span not found, this is a bug");
  + let mut extensions = span.extensions_mut();
  + let builder = extensions
  + .get_mut::<api::SpanBuilder>()
  + .expect("Missing SpanBuilder span extensions");
  +
  + let follows_span = ctx
  + .span(follows)
  + .expect("Span to follow not found, this is a bug");
  + let mut follows_extensions = follows_span.extensions_mut();
  + let follows_builder = follows_extensions
  + .get_mut::<api::SpanBuilder>()
  + .expect("Missing SpanBuilder span extensions");
  +
  + let follows_context = build_span_context(follows_builder, self.sampler.as_ref());
  + let follows_link = api::Link::new(follows_context, Vec::new());
  + if let Some(ref mut links) = builder.links {
  + links.push(follows_link);
  + } else {
  + builder.links = Some(vec![follows_link]);
  + }
  + }
  +
  /// Records OpenTelemetry [`Event`] data on event.
  ///
  /// Note: an [`ERROR`]-level event will also set the OpenTelemetry span status code to
  