Add log_injection option to auto enable rails trace/log correlation

[ericmustin]

Summary

This PR adds an option to the rails instrumentation, log_injection that, when enabled automatically adds trace correlation information to Rails logs when using one of the Datadog supported loggers, ActiveSupport::TaggedLogging or Lograge. This option can also be enabled with the environment variable DD_LOGS_INJECTION=true.

Additionally, this PR updates the GettingStarted documentation to more effectively document the onboarding setup for Trace Correlation with logs, by changing the order of our suggested approaches and also incorporating automatic injection.

Notes

• The work here attempts to ensure that any current configuration of ActiveSupport::TaggedLogging or Lograge is maintained and merged in with our trace correlation information.

• In the event of an active_trace not existing, we still inject the relevant keys but leave the values blank. This is a standard practice across our other language Log Injection Implementation at Datadog.

• I tried to document this clearly but definitely could use a second opinion.

• Testing wise It was a huge hassle to test log output effectively, the approach I took is a little hacky (originally i was logging to a file but there was leakage between tests), but i think it should work and be robust enough to not leak across tests. I borrowed this approach from the actual Lograge testing setup, https://github.com/roidrage/lograge/blob/1729eab7956bb95c5992e4adab251e4f93ff9280/spec/log_subscribers/action_controller_spec.rb#L9-L35

• I added the gem in all the appraisals, i'm not 100% sure how far back Lograge and ActiveSupport::TaggedLogging go in terms of rails, looks like https://www.rubydoc.info/docs/rails/3.2.8/ActiveSupport/TaggedLogging 3.2.8 for Tagged Logging but not sure about Lograge.

[marcotc]

I made a few changes to the documentation as well, please feel free to modify them if you think there's something that needs to be changed in it.

[marcotc]

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