Expose coverage on GET pipelines/:id

The coverage wasn't exposed yet, now it is but only for detailed
requests to save queries on the database.

changelogs/unreleased/zj-expose-coverage-pipelines.yml

+---
+title: 'API: expose pipeline coverage'
+merge_request: 
+author: 

lib/api/entities.rb

@@ -606,6 +606,7 @@ module API
       expose :user, with: Entities::UserBasic
       expose :created_at, :updated_at, :started_at, :finished_at, :committed_at
       expose :duration
+      expose :coverage
     end
 
     class EnvironmentBasic < Grape::Entity

spec/requests/api/pipelines_spec.rb

@@ -103,6 +103,18 @@ describe API::API, api: true do
         expect(json_response['message']).to eq '404 Not found'
         expect(json_response['id']).to be nil
       end
+
+      context 'with coverage' do
+        before do
+          create(:ci_build, coverage: 30, pipeline: pipeline)
+        end
+
+        it 'exposes the coverage' do
+          get api("/projects/#{project.id}/pipelines/#{pipeline.id}", user)
+
+          expect(json_response["coverage"].to_i).to eq(30)
+        end
+      end
     end
 
     context 'unauthorized user' do