Rails: ActiveSupport notification subscriptions should be spans

[nateberkopec]

Currently, a lot of ActiveSupport notifications are used to create breadcrumbs

I think pretty much all of these would also be useful spans, particularly the cache stuff.

[frederikspang]

Cache is being worked on in #2380 - Not as breadcrumb :)

[sl0thentr0py]

thx @nateberkopec, we could definitely update our list to include more spans. I'm a bit wary of including everything because

• we had serialization problems in the past with some of them
• in other languages, we've had some spans that just act as noise so I'd like to avoid high volume stuff