Add "multi" methods instrumentation for Rails cache integration

[michaelkl]

In current implementation only read, write and fetch methods of ActiveSupport::Cache::Store are instrumented. This PR adds the instrumentation of read_multi, write_multi and fetch_multi methods.

[ericmustin]

👋 @michaelkl thanks for the contribution! Generally this looks like a really solid addition to the library. I am a bit backlogged right now but should be able to give this a thoughtful review in the coming days, I want to review how some other libraries and languages handle multi read/write/fetch operations, But i think the approach here makes sense.

Also, for the linting failed tests, bundle exec rake rubocop from the root of the dd-trace-rb aught to surface any linting issues

[michaelkl]

Thank you @ericmustin ! Glad to help the community!
I tried to follow the same approach already used for existing cache operations.

I will remove the Draft status from the PR when it turns out ready to be reviewed. Take your time, no rush at all!

[marcotc]

Great work @michaelkl! I left a few suggestion on my review, but it looks very close to completion. Great job on getting it to pass tests on all (even very old) versions of Rails 👍

[marcotc]

Thank you very much, @michaelkl! 🚀