ActiveSupport::Notifications Support

[wagenet]

Is there any objection to adding ActiveSupport::Notifications instrumentation? It looks like it existed briefly in 0.9 but is no longer in master. I'd be happy to help with this for use in Skylight.

[beauby]

No objection. Possibly related to #1291.

[bf4]

Sure @wagenet you wanna make a PR?

[wagenet]

Looks like #1291 would actually address my use case. Missed it in earlier searching.