[rails] Rails auto-instrumentation is disabled by default

[palazzem]

What it does

Disables Rails auto-instrumentation by default. This means that adding the gem to the Gemfile is not enough to enable the tracing. Users must add the initializer file so that:

# config/initializers/datadog-tracer.rb

Rails.configuration.datadog_trace = {
  # these two are required now otherwise the
  # instrumentation will not be active
  auto_instrument: true,
  auto_instrument_redis: true,

  default_service: 'my-rails-app'
}

If users want to activate the tracing only for some environments, they may:

# config/initializers/datadog-tracer.rb

# this is an example; users may not have these environments
tracer_status = Rails.env.staging? || Rails.env.production?

Rails.configuration.datadog_trace = {
  auto_instrument: tracer_status,
  auto_instrument_redis: tracer_status,
  default_service: 'my-rails-app'
}

In this way, is up to users in which environment enable the Rails auto-instrumentation, so that installing our gem will not have any kind of side-effect until users enable the gem.
As a side note, the default tracer (Datadog.tracer) will still be activated and the regular tracer can be used.

[LotharSee]

This URL is very likely to change, let's avoid linking app-internal URL in the doc. Maybe as my-rails-app or as my-rails-app in your service list could do it.

[palazzem]

👍