Add tests for Thread instrumentation using Rails

[kaylareopelle]

Create a simple Rails controller that makes external requests using Net::HTTP or another instrumented HTTP Client within threads. Turn on our new Thread instrumentation, introduced in #1080, and make sure the spans have the expected relational structure and are created as we saw them in our playground environment.

T-Shirt size

[angelatan2]

We have unit test coverage with thread instrumentation. Lower priority.

[workato-integration[bot]]

https://issues.newrelic.com/browse/NEWRELIC-3465

[fallwith]

Addressed with subsequent threading work done in v9.x.