Add OTel span attribute to deprecated endpoints (#40030)

app/controllers/concerns/deprecation_concern.rb

@@ -11,6 +11,11 @@ module DeprecationConcern
       before_action(**kwargs) do
         response.headers['Deprecation'] = deprecation_timestamp
         response.headers['Sunset'] = sunset if sunset
+
+        span = OpenTelemetry::Trace.current_span
+        next unless span&.recording?
+
+        span.set_attribute('app.endpoint.deprecated', true)
       end
     end
   end

spec/controllers/concerns/deprecation_concern_spec.rb

@@ -0,0 +1,54 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe DeprecationConcern do
+  render_views
+
+  controller(ApplicationController) do
+    include DeprecationConcern # rubocop:disable RSpec/DescribedClass
+
+    deprecate_api '2026-08-03', sunset: '2026-10-01'
+
+    def deprecated
+      render plain: 'deprecated'
+    end
+  end
+
+  before do
+    routes.draw do
+      get 'deprecated' => 'anonymous#deprecated'
+    end
+  end
+
+  context 'with response headers' do
+    it 'adds Deprecation header' do
+      get :deprecated
+
+      expect(response.body).to eq('deprecated')
+      expect(response.headers['Deprecation']).to eq('@1785715200')
+    end
+
+    it 'adds Sunset header' do
+      get :deprecated
+
+      expect(response.headers['Sunset']).to eq('Thu, 01 Oct 2026 00:00:00 GMT')
+    end
+  end
+
+  context 'with OpenTelemetry traces' do
+    let(:span) { instance_double(OpenTelemetry::Trace::Span) }
+
+    before do
+      allow(OpenTelemetry::Trace).to receive(:current_span).and_return(span)
+      allow(span).to receive(:recording?).and_return(true)
+      allow(span).to receive(:set_attribute)
+    end
+
+    it 'adds a deprecation attribute request span' do
+      get :deprecated
+
+      expect(span).to have_received(:set_attribute).with('app.endpoint.deprecated', true)
+    end
+  end
+end