fix: add Active Support as runtime dependency in Racecar instrumentation

[razumau]

Installation of Racecar instrumentation silently fails without Active Support. Since this instrumentation doesn’t work at all without Active Support, it should be listed as a runtime dependency.

It was already being installed in test and example Gemfiles.

[linux-foundation-easycla]

• ✅login: razumau / name: Jury Razumau / (d333db9)

The committers listed above are authorized under a signed CLA.

[arielvalentin]

We explicitly avoid adding runtime dependencies in instrumentations because we do not cause conflicts or bring in unexpected dependencies into the application runtime.

The racecar gem itself does not add it as a runtime dependency either, but exposes all of its instrumentation using the active support API, which is why this instrumentation hooks into those notifications for instrumentation purposes:

• https://github.com/zendesk/racecar/blob/79764adfec009f814edf899aedd0dc6196e285cd/racecar.gemspec#L34
• https://github.com/zendesk/racecar/blob/79764adfec009f814edf899aedd0dc6196e285cd/lib/racecar/config.rb#L286

There isn't much to go by here nor do they publish docs and what a user should do. zendesk/racecar#43

I would recommend engaging the Zendesk engineers to see if they could provide more explicit guidance for instrumentation as opposed to having our libraries unexpectedly adding additional 3rd party gems.