Expose `duration` in Job API entity

Closes #35794.

changelogs/unreleased/expose-job-duration.yml

+---
+title: Expose duration in Job entity
+merge_request: 13644
+author: Mehdi Lahmam (@mehlah)
+type: added

lib/api/entities.rb

@@ -822,6 +822,7 @@ module API
     class Job < Grape::Entity
       expose :id, :status, :stage, :name, :ref, :tag, :coverage
       expose :created_at, :started_at, :finished_at
+      expose :duration
       expose :user, with: User
       expose :artifacts_file, using: JobArtifactFile, if: -> (job, opts) { job.artifacts? }
       expose :commit, with: Commit

spec/requests/api/jobs_spec.rb

@@ -165,7 +165,17 @@ describe API::Jobs do
     context 'authorized user' do
       it 'returns specific job data' do
         expect(response).to have_gitlab_http_status(200)
-        expect(json_response['name']).to eq('test')
+        expect(json_response['id']).to eq(job.id)
+        expect(json_response['status']).to eq(job.status)
+        expect(json_response['stage']).to eq(job.stage)
+        expect(json_response['name']).to eq(job.name)
+        expect(json_response['ref']).to eq(job.ref)
+        expect(json_response['tag']).to eq(job.tag)
+        expect(json_response['coverage']).to eq(job.coverage)
+        expect(Time.parse(json_response['created_at'])).to be_like_time(job.created_at)
+        expect(Time.parse(json_response['started_at'])).to be_like_time(job.started_at)
+        expect(Time.parse(json_response['finished_at'])).to be_like_time(job.finished_at)
+        expect(json_response['duration']).to eq(job.duration)
       end
 
       it 'returns pipeline data' do