Merge branch 'feature/expose-timestimes-in-build-entity' into 'master'

Expose timestamps in build entity

## What does this MR do?

This MR exposes timestamps for build entity that is being used by serializers in `app/serializers`. 

## Does this MR meet the acceptance criteria?

- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing

## What are the relevant issue numbers?

See https://gitlab.com/gitlab-org/gitlab-ce/issues/24844#note_19145053

See merge request !7818

app/serializers/build_entity.rb

@@ -16,6 +16,9 @@ class BuildEntity < Grape::Entity
     path_to(:play_namespace_project_build, build)
   end
 
+  expose :created_at
+  expose :updated_at
+
   private
 
   def path_to(route, build)

spec/serializers/build_entity_spec.rb

 require 'spec_helper'
 
 describe BuildEntity do
+  let(:build) { create(:ci_build) }
+
   let(:entity) do
     described_class.new(build, request: double)
   end
 
   subject { entity.as_json }
 
-  context 'when build is a regular job' do
-    let(:build) { create(:ci_build) }
-
   it 'contains paths to build page and retry action' do
     expect(subject).to include(:build_path, :retry_path)
-      expect(subject).not_to include(:play_path)
   end
 
   it 'does not contain sensitive information' do
     expect(subject).not_to include(/token/)
     expect(subject).not_to include(/variables/)
   end
+
+  it 'contains timestamps' do
+    expect(subject).to include(:created_at, :updated_at)
+  end
+
+  context 'when build is a regular job' do
+    it 'does not contain path to play action' do
+      expect(subject).not_to include(:play_path)
+    end
   end
 
   context 'when build is a manual action' do