Explicitly require format.json in prometheus_controller

+ add missing prometheus_controller tests!

app/controllers/projects/deployments_controller.rb

@@ -24,10 +24,11 @@ class Projects::DeploymentsController < Projects::ApplicationController
 
   def additional_metrics
     return render_404 unless deployment.has_additional_metrics?
+
     metrics = deployment.additional_metrics
 
-    if metrics&.any?
-      render json: metrics, status: :ok
+    if metrics.any?
+      render json: metrics
     else
       head :no_content
     end

app/controllers/projects/prometheus_controller.rb

@@ -3,17 +3,25 @@ class Projects::PrometheusController < Projects::ApplicationController
   before_action :require_prometheus_metrics!
 
   def active_metrics
-    matched_metrics = prometheus_service.reactive_query(Gitlab::Prometheus::Queries::MatchedMetricsQuery.name, &:itself)
+    matched_metrics = prometheus_service.reactive_query(Gitlab::Prometheus::Queries::MatchedMetricsQuery.name, &:itself) || {}
 
-    if matched_metrics
-      render json: matched_metrics, status: :ok
-    else
-      head :no_content
+    respond_to do |format|
+      format.json do
+        if matched_metrics.any?
+          render json: matched_metrics
+        else
+          head :no_content
+        end
+      end
     end
   end
 
   private
 
+  rescue_from(ActionController::UnknownFormat) do |e|
+    render_404
+  end
+
   def prometheus_service
     project.monitoring_service
   end

spec/controllers/projects/prometheus_controller_spec.rb

@@ -0,0 +1,60 @@
+require('spec_helper')
+
+describe Projects::PrometheusController do
+  let(:user) { create(:user) }
+  let!(:project) { create(:empty_project) }
+
+  let(:prometheus_service) { double('prometheus_service') }
+
+  before do
+    allow(controller).to receive(:project).and_return(project)
+    allow(project).to receive(:monitoring_service).and_return(prometheus_service)
+
+    project.add_master(user)
+    sign_in(user)
+  end
+
+  describe 'GET #active_metrics' do
+    context 'when prometheus metrics are enabled' do
+      before do
+        allow(prometheus_service).to receive(:reactive_query)
+      end
+
+      context 'when data is not present' do
+        it 'returns no content response' do
+          get :active_metrics, project_params(format: :json)
+
+          expect(response).to have_http_status(204)
+        end
+      end
+
+      context 'when data is available' do
+        let(:sample_response) { { some_data: 1 } }
+
+        before do
+          allow(prometheus_service).to receive(:reactive_query).with(Gitlab::Prometheus::Queries::MatchedMetricsQuery.name)
+                                         .and_return(sample_response)
+        end
+
+        it 'returns no content response' do
+          get :active_metrics, project_params(format: :json)
+
+          expect(response).to have_http_status(200)
+          expect(json_response).to eq(sample_response.deep_stringify_keys)
+        end
+      end
+
+      context 'when requesting non json response' do
+        it 'returns not found response' do
+          get :active_metrics, project_params
+
+          expect(response).to have_http_status(404)
+        end
+      end
+    end
+  end
+
+  def project_params(opts = {})
+    opts.reverse_merge(namespace_id: project.namespace, project_id: project)
+  end
+end