ActiveModelSerializer instrumentation does not make the span current

[renchap]

Description of the bug

Spans occuring during a render call using AM:S are not set as children span of the AM:S render span

Here, the spans below the serializer are made by the serializer code.

Share details about your runtime

Operating system details: Linux, Ubuntu 20.04 LTS
RUBY_ENGINE: "ruby"
RUBY_VERSION: "3.2.2"
RUBY_DESCRIPTION: "ruby 3.2.2 (2023-03-30 revision e51014f9c0) [arm64-darwin22]"

Share a simplified reproduction if possible

No reproduction, but the issue is that the event handler is setting itself as the current span here: https://github.com/open-telemetry/opentelemetry-ruby-contrib/blob/main/instrumentation/active_model_serializers/lib/opentelemetry/instrumentation/active_model_serializers/event_handler.rb#L14-L20

Pinging @robbkidd as he helped me on this

[robbkidd]

Thanks for opening this, @renchap!

My hypothesis is that ActiveModelSerializers instrumentation is using an older style of AS::Notification event subscription and handling. It does not start the span in a way that makes the serialization span the current span while it is doing work. (And it should.)