Render 404 when prometheus adapter is disabled in Prometheus metrics controller

app/controllers/projects/prometheus/metrics_controller.rb

@@ -25,7 +25,7 @@ module Projects
       end
 
       def require_prometheus_metrics!
-        render_404 unless prometheus_adapter.can_query?
+        render_404 unless prometheus_adapter&.can_query?
       end
     end
   end

changelogs/unreleased/46413-error-when-prometheus-adapter-is-disabled.yml

+---
+title: Render 404 when prometheus adapter is disabled in Prometheus metrics controller
+merge_request: 19110
+author:
+type: fixed

spec/controllers/projects/prometheus/metrics_controller_spec.rb

@@ -12,6 +12,7 @@ describe Projects::Prometheus::MetricsController do
   end
 
   describe 'GET #active_common' do
+    context 'when prometheus_adapter can query' do
       before do
         allow(controller).to receive(:prometheus_adapter).and_return(prometheus_adapter)
       end
@@ -54,6 +55,28 @@ describe Projects::Prometheus::MetricsController do
       end
     end
 
+    context 'when prometheus_adapter cannot query' do
+      it 'renders 404' do
+        prometheus_adapter = double('prometheus_adapter', can_query?: false)
+
+        allow(controller).to receive(:prometheus_adapter).and_return(prometheus_adapter)
+        allow(prometheus_adapter).to receive(:query).with(:matched_metrics).and_return({})
+
+        get :active_common, project_params(format: :json)
+
+        expect(response).to have_gitlab_http_status(404)
+      end
+    end
+
+    context 'when prometheus_adapter is disabled' do
+      it 'renders 404' do
+        get :active_common, project_params(format: :json)
+
+        expect(response).to have_gitlab_http_status(404)
+      end
+    end
+  end
+
   describe '#prometheus_adapter' do
     before do
       allow(controller).to receive(:project).and_return(project)