Exclude coverage data from the pipelines page

When displaying a project's pipelines
(Projects::PipelinesController#index) we now exclude the coverage data.
This data was not used by the frontend, yet getting it would require one
SQL query per pipeline. These queries in turn could be quite expensive
on GitLab.com.

app/controllers/projects/pipelines_controller.rb

@@ -34,7 +34,7 @@ class Projects::PipelinesController < Projects::ApplicationController
           pipelines: PipelineSerializer
             .new(project: @project, current_user: @current_user)
             .with_pagination(request, response)
-            .represent(@pipelines),
+            .represent(@pipelines, disable_coverage: true),
           count: {
             all: @pipelines_count,
             running: @running_count,

app/serializers/pipeline_entity.rb

@@ -4,7 +4,11 @@ class PipelineEntity < Grape::Entity
   expose :id
   expose :user, using: UserEntity
   expose :active?, as: :active
-  expose :coverage
+
+  # Coverage isn't always necessary (e.g. when displaying project pipelines in
+  # the UI). Instead of creating an entirely different entity we just allow the
+  # disabling of this specific field whenever necessary.
+  expose :coverage, unless: proc { options[:disable_coverage] }
   expose :source
   expose :created_at, :updated_at

spec/controllers/projects/pipelines_controller_spec.rb

@@ -41,6 +41,12 @@ describe Projects::PipelinesController do
       expect(json_response['count']['finished']).to eq '1'
     end
+    it 'does not include coverage data for the pipelines' do
+      subject
+
+      expect(json_response['pipelines'][0]).not_to include('coverage')
+    end
+
     context 'when performing gitaly calls', :request_store do
       it 'limits the Gitaly requests' do
         expect { subject }.to change { Gitlab::GitalyClient.get_request_count }.by(3)

spec/serializers/pipeline_entity_spec.rb

@@ -26,6 +26,13 @@ describe PipelineEntity do
         expect(subject).to include :updated_at, :created_at
       end
+      it 'excludes coverage data when disabled' do
+        entity = described_class
+          .represent(pipeline, request: request, disable_coverage: true)
+
+        expect(entity.as_json).not_to include(:coverage)
+      end
+
       it 'contains details' do
         expect(subject).to include :details
         expect(subject[:details])