Add ActiveStorage Instrumentation

[cwoodcox]

An updated version of #1290, this adds ActiveStorage instrumentation via ActiveSupport::Notifications just like the other Rails integrations.

To-do

• add @public_api tags to the proper methods/modules to match the rest
• add documentation comments for the individual event modules
• change span_name in all events to be the event name (delete, exist, etc)
• change resource name to be blob name
  • include storage service name?
• add tests

[marcotc]

This is an awesome PR, super flushed out already, thank you @cwoodcox! 🙇
With small tweaks and testing this can be merged very soon.

I left an inline comment for us to discuss.

Besides testing, one thing missing is user-facing documentation, which goes in the https://github.com/DataDog/dd-trace-rb/blob/master/docs/GettingStarted.md file. You can certainly leave it until the end to add it though.

[marcotc]

I believe as_key has high cardinality, as it seems to be unique per file/path stored.
This falls under the same unresolved issue we have with instrumenting URL paths for arbitrary URLs: there's no reliable default quantizer we can think of that wouldn't cause high cardinality for some users.

I recommend the removal of as_key here, and instead set it as a tag (span.set_tag(...,as_key)).

Please let me know if my assumptions are incorrect and as_key is either a predictable value or can be quantized reliably.

[ivoanjo]

This PR has been open for quite a while; @cwoodcox is this something you're still interested in pushing forward?