BAU: configure OpenTelemetry ID Generator

We need to use the XRay ID generator when using the XRay propagator, in
order to generate trace IDs in the correct format. As we're configuring
the propagator via the OTEL_PROPAGATORS environment variable, we can
gate the ID generator configuration behind the same variable to keep
things agnostic.

This can only be done via code, so we have to add this to the
OpenTelemetry SDK configuration in an initializer.
see: https://aws-otel.github.io/docs/getting-started/ruby-sdk/trace-manual-instr#instrumenting-an-application:~:text=Next%2C%20because%20the%20AWS%20X%2DRay%20ID%20Generator%20can%20only%20be%20configured%20through%20code%2C%20you%20cannot%20use%20an%20environment%20variable%20to%20select%20it.%20The%20ID%20Generator%20must%20be%20used%20at%20the%20time%20OpenTelemetry%20Ruby%20SDK%20is%20configured

Author: whi-tw

config/initializers/opentelemetry.rb

@@ -7,6 +7,11 @@
   instrumentation_config = { "OpenTelemetry::Instrumentation::Rack" => { untraced_endpoints: ["/up"] } }
   c.use_all(instrumentation_config)
 
+  if ENV["OTEL_PROPAGATORS"] == "xray"
+    # The ID Generator can only be configured through code. Gate it behind the propagator env var to keep things agnostic.
+    c.id_generator = OpenTelemetry::Propagator::XRay::IDGenerator
+  end
+
   # Disable logging for Rake tasks to avoid cluttering output
   c.logger = Logger.new(File::NULL) if Rails.const_defined?(:Rake) && Rake.application.top_level_tasks.any?
 end