Rails 6 support

[marcotc]

Add support for all Rails tracer features to Rails 6.

Compared to Rails 5, there are not new features in Rails 6 that require special instrumentation for enhanced visibility.

One example is support for multiple databases: the infrastructure was already present in its current form since Rails 5.

Also, a few notable changes to our testing infrastructure:

• contrib Rails tests now run on Rails 6.
• Bumped sqlite3 version to 1.4: Rails 6 requires it and our reason to lock it to 1.3 doesn't hold anymore as Rails has released new versions that are compatible with sqlite3 1.4.
• Avoid loading Rails submodules (Active*/Action* gems) before application initialization by loading only routes routes first, and loading controllers after initialization. This is recommended for Rails 6 with its new autoloader, zeitwerk, and will be mandatory in a future version.

[marcotc]

After many iterations, I ended up with separate modules to encapsulate this branching logic between Rails v3.1-v5 and v6.

The reason I chose this approach in end was because:

• It makes it clear to us which implementation is the latest, without explicit branching logic.
  • This allows for easier deprecation of older implementations in the future.
• It avoids conditional branching at runtime, making such decision during our patching phase.

I did add a method call, so the overhead of this approach is not zero.

In the end, I valued how clearly Rails 6 and future versions will read over succinctness of changes (few lines touched), as my best judgement here is that this approach increases maintainability.

Regardless of my rationale, I'm very much open to suggestions.

[delner]

Going to give this a more thorough review later, but I like what I see so far! One minor comment related to tests on the first scan.

[delner]

Looks pretty good! Just want to request some relatively small housekeeping tasks, but otherwise very pleased with this.

[delner]

This Application class is defined as a full-blown constant (as opposed to some kind of stub) because of how we setup Minitest to test Rails yeah? I don't think there's anything else to do here, just wanted to clarify that's the case, which would mean this will go away when we finish migrating to RSpec.

[marcotc]

This is correct!

[delner]

Just one suggestion about naming, otherwise this is ready.

[delner]

Nitpick but I think it should be Instrumentation singular.

[delner]

Looks great, thanks @marcotc! 👍