Load environment metrics only for JSON endpoint

When showing the HTML version of the environment metrics we don't need
to fetch their metrics because we don't use them anymore on the HTML
version.

We use additional_metrics.json endpoint now.

app/controllers/projects/environments_controller.rb

@@ -135,13 +135,13 @@ class Projects::EnvironmentsController < Projects::ApplicationController
   end
 
   def metrics
-    # Currently, this acts as a hint to load the metrics details into the cache
-    # if they aren't there already
-    @metrics = environment.metrics || {}
-
     respond_to do |format|
       format.html
       format.json do
+        # Currently, this acts as a hint to load the metrics details into the cache
+        # if they aren't there already
+        @metrics = environment.metrics || {}
+
         render json: @metrics, status: @metrics.any? ? :ok : :no_content
       end
     end

spec/controllers/projects/environments_controller_spec.rb

@@ -342,11 +342,9 @@ describe Projects::EnvironmentsController do
     end
 
     context 'when environment has no metrics' do
-      before do
-        expect(environment).to receive(:metrics).and_return(nil)
-      end
-
       it 'returns a metrics page' do
+        expect(environment).not_to receive(:metrics)
+
         get :metrics, params: environment_params
 
         expect(response).to be_ok
@@ -354,6 +352,8 @@ describe Projects::EnvironmentsController do
 
       context 'when requesting metrics as JSON' do
         it 'returns a metrics JSON document' do
+          expect(environment).to receive(:metrics).and_return(nil)
+
           get :metrics, params: environment_params(format: :json)
 
           expect(response).to have_gitlab_http_status(204)