Add spec for invalid environment id

app/controllers/projects/environments/prometheus_api_controller.rb

@@ -15,11 +15,10 @@ class Projects::Environments::PrometheusApiController < Projects::ApplicationCon
     ).execute
 
     if result.nil?
-      render status: :accepted, json: {
+      return render status: :accepted, json: {
         status: 'processing',
         message: 'Not ready yet. Try again later.'
       }
-      return
     end
 
     if result[:status] == :success

spec/controllers/projects/environments/prometheus_api_controller_spec.rb

@@ -124,6 +124,16 @@ describe Projects::Environments::PrometheusApiController do
         end
       end
     end
+
+    context 'with invalid environment id' do
+      let(:other_environment) { create(:environment) }
+
+      it 'returns 404' do
+        get :proxy, params: environment_params(id: other_environment.id)
+
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+    end
   end
 
   private