fix: Backport Rack proxy event to middleware

instrumentation/rack/lib/opentelemetry/instrumentation/rack/middlewares/tracer_middleware.rb

@@ -77,6 +77,8 @@ def call(env)
               tracer.in_span(request_span_name,
                              attributes: request_span_attributes(env: env),
                              kind: request_span_kind) do |request_span|
+                request_start_time = OpenTelemetry::Instrumentation::Rack::Util::QueueTime.get_request_start(env)
+                request_span.add_event('http.proxy.request.started', timestamp: request_start_time) unless request_start_time.nil?
                 OpenTelemetry::Instrumentation::Rack.with_span(request_span) do
                   @app.call(env).tap do |status, headers, response|
                     set_attributes_after_request(request_span, status, headers, response)

instrumentation/rack/test/opentelemetry/instrumentation/rack/middlewares/tracer_middleware_test.rb

@@ -25,6 +25,7 @@
   let(:exporter) { EXPORTER }
   let(:finished_spans) { exporter.finished_spans }
   let(:first_span) { exporter.finished_spans.first }
+  let(:proxy_event) { first_span.events&.first }
 
   let(:default_config) { {} }
   let(:config) { default_config }
@@ -84,6 +85,15 @@
       end
     end
 
+    describe 'given request proxy headers' do
+      let(:env) { Hash('HTTP_X_REQUEST_START' => '1677723466') }
+
+      it 'records an event' do
+        _(proxy_event.name).must_equal 'http.proxy.request.started'
+        _(proxy_event.timestamp).must_equal 1_677_723_466_000_000_000
+      end
+    end
+
     describe 'config[:untraced_endpoints]' do
       describe 'when an array is passed in' do
         let(:config) { { untraced_endpoints: ['/ping'] } }