Simplify instrumentation

[kikorb]

We want to simplify the instrumentation and the way it is configured:

Before

Opera::Operation::Config.configure do |config|
  config.instrumentation_class = Datadog::Tracing
  config.instrumentation_method = :trace
  config.instrumentation_options = { service: :operation }
end

After

Opera::Operation::Config.configure do |config|
  config.instrumentation_class = OperaDatadogTracing
end