Explore additional Rails instrumentation

[ahayworth]

As we switch to ActiveSupport::Notifications for instrumentation in #218, there are additional other instrumentations we could easily add, if they are useful:

• ActionMailer instrumentation #293
• cache stuff:https://guides.rubyonrails.org/active_support_instrumentation.html#active-support
  • Sadly that's part of ActiveSupport, and we already have an instrumentation gem for that ... which is really not instrumenting anything, and is instead a dependency. We'd need to disentangle that. 🤔
• actioncable: https://guides.rubyonrails.org/active_support_instrumentation.html#action-cable
• activestorage: https://guides.rubyonrails.org/active_support_instrumentation.html#active-storage

cc @arielvalentin

[github-actions]

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[ymtdzzz]

Hi, I'm interested in adding instrumentation for ActiveStorage. Can I work on it?
If it's ok, I will create an issue and then get to work :)