Add ActiveRecord cached tag

[delner]

Add the cached tag to ActiveRecord query spans that are cached.

The old implementation was not correctly tagging ActiveRecord queries that were resolved through the query cache. This pull requests fixes that implementation, adds the same tag for ActiveRecord integrations outside of Rails, and adds test coverage for both scenarios.

Still need to ascertain what milestone this should be applied to.

[p-lambert]

Great job! 👏
CI is breaking though – waiting for you to fix it

[delner]

Rubocop strikes again!

[delner]

Test failure was some kind of race condition, I think. Writer wasn't dumping its spans before they were asserted upon. Weirdly enough, regardless of how I used try_wait_until, it would sometimes intermittently fail anyways. (e.g., made it attempt 100 times, it would wait for like 10 seconds, then still fail.) Worker threads seemed alive but sleeping. Locked out by mutex or something?

I avoided this altogether by moving my caching ActiveRecord queries into a Sinatra endpoint, and then querying that endpoint to do the AR queries, instead of doing the AR queries directly in my test like I did previously. I honestly don't know why this fixed it, maybe something configuration related, but it seemed to eliminate the race condition for the test, and the need for try_wait_until.

[palazzem]

Good to me, but we need to fix the test suite first.

[delner]

Seems to be failing specifically on the old versions of Ruby (1.9.3, 2.0.0, 2.1.10), ActiveRecord (3.2) and Sinatra (1.4.5.)

rvm $MRI_OLD_VERSIONS --verbose do appraisal contrib-old rake test:sinatra

Digging deeper into this.

[delner]

Turns out my assertion didn't match output from different versions of ActiveRecord. Very minor, easy thing to fix, and an oversight on my part.