refactor: unify rack middleware_args

[sergiofbsilva]

Standalone rack applications need to initialize middlewares explicitly:

Rack::Builder.new do
	use *OpenTelemetry::Instrumentation::Rack::Instrumentation.instance.middleware_args
	run lambda { |_arg| [200, { 'Content-Type' => 'text/plain' }, body] }
 end

However this doesn't take into account the new OTEL_SEMCONV_STABILITY_OPT_IN environment variable.

This PR ensures rack middleware_args are selected consistently based on OTEL_SEMCONV_STABILITY_OPT_IN environment variable.

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[ericmustin]

Thanks for opening this @sergiofbsilva . We chatted on this briefly in SIG, just wanted to clarify

• To confirm, the issue here that you're using rack as a standalone app, and the env var OTEL_SEMCONV_STABILITY_OPT_IN isn't being respected because it isn't being by the evaluated by the middleware_args helper method?

cc @hannahramadan

[sergiofbsilva]

Thanks for opening this @sergiofbsilva . We chatted on this briefly in SIG, just wanted to clarify

• To confirm, the issue here that you're using rack as a standalone app, and the env var OTEL_SEMCONV_STABILITY_OPT_IN isn't being respected because it isn't being by the evaluated by the middleware_args helper method?

cc @hannahramadan

Yes, that's right. We want to use middleware_args without all the logic around OTEL_SEMCONV_STABILITY_OPT_IN being on the API client.

[hannahramadan]

Thanks @sergiofbsilva! This looks like a good change, and allows users to continuing using middleware_args vs needing to explicitly add one of the new middleware_args_* methods. We look forward to reviewing when you're ready.