ActiveStorage instrumentation

[ymtdzzz]

related: #298

We can add instrumentation for ActiveStorage by subscribing to *.active_storage events.
https://guides.rubyonrails.org/active_support_instrumentation.html#active-storage

The ActionMailer instrumentation could be helpful.
https://github.com/open-telemetry/opentelemetry-ruby-contrib/tree/main/instrumentation/action_mailer

When I play around with these events, I feel like we can get useful information.

[ymtdzzz]

I have created a PR to add ActiveStorage instrumentation and would be happy to review it when someone has time :)