fix: Backport Rack proxy event to middleware

[fbogsany]

The Rack Event handler instrumentation adds a Span Event representing the time the request was enqueued. See

opentelemetry-ruby-contrib/instrumentation/rack/lib/opentelemetry/instrumentation/rack/middlewares/event_handler.rb

Lines 263 to 264 in 1fa0810

	request_start_time = OpenTelemetry::Instrumentation::Rack::Util::QueueTime.get_request_start(request.env)
	span.add_event('http.proxy.request.started', timestamp: request_start_time) unless request_start_time.nil?

The older middleware-based instrumentation represents the same information (optionally enabled via config[:record_frontend_span]) as a http_server.proxy span as a "fake" parent span for the Rack server span. The Span Event approach is both more correct and more useful for analysis.

This PR "backports" the Span Event approach to the middleware instrumentation. Since this is done unconditionally in the Rack Event handler instrumentation, I've made it unconditional in the middleware as well. If the config[:record_frontend_span] option is enabled, this will duplicate information, but I would argue we should deprecate that config option (and the corresponding code).

Many thanks to @arielvalentin for writing great code that I can steal to inspire me.