added analytics stage serializer and moved some info to the stage classes from the controller

app/serializers/analytics_stage_entity.rb

+class AnalyticsStageEntity < Grape::Entity
+  include EntityDateHelper
+
+  expose :stage, as: :title do |object|
+    object[:stage].to_s.capitalize
+  end
+  expose :description
+
+  expose :median, as: :value do |stage|
+    stage[:median] && !stage[:median].zero? ? distance_of_time_in_words(stage[:median]) : nil
+  end
+end

app/serializers/analytics_stage_serializer.rb

+class AnalyticsStageSerializer < BaseSerializer
+  entity AnalyticsStageEntity
+end

lib/gitlab/cycle_analytics/base_stage.rb

 module Gitlab
   module CycleAnalytics
     class BaseStage
+      attr_reader :stage, :description
+
       def initialize(project:, options:, stage: stage)
         @project = project
         @options = options

lib/gitlab/cycle_analytics/code_stage.rb

 module Gitlab
   module CycleAnalytics
     class CodeStage < BaseStage
+      def initialize(*args)
+        super(*args)
+
+        @description = "Time until first merge request"
+      end
+
       def median
         @fetcher.calculate_metric(:code,
                                   Issue::Metrics.arel_table[:first_mentioned_in_commit_at],

lib/gitlab/cycle_analytics/issue_stage.rb

 module Gitlab
   module CycleAnalytics
     class IssueStage < BaseStage
+      def initialize(*args)
+        super(*args)
+
+        @description = "Time before an issue gets scheduled"
+      end
+
       def median
         @fetcher.calculate_metric(:issue,
                                   Issue.arel_table[:created_at],

lib/gitlab/cycle_analytics/plan_stage.rb

 module Gitlab
   module CycleAnalytics
     class PlanStage < BaseStage
+      def initialize(*args)
+        super(*args)
+
+        @description = "Time before an issue starts implementation"
+      end
+
       def median
         @fetcher.calculate_metric(:plan,
                                   [Issue::Metrics.arel_table[:first_associated_with_milestone_at],

lib/gitlab/cycle_analytics/production_stage.rb

 module Gitlab
   module CycleAnalytics
     class ProductionStage < BaseStage
+      def initialize(*args)
+        super(*args)
+
+        @description = "From issue creation until deploy to production"
+      end
+
       def median
         @fetcher.calculate_metric(:production,
                                   Issue.arel_table[:created_at],

lib/gitlab/cycle_analytics/review_stage.rb

 module Gitlab
   module CycleAnalytics
     class ReviewStage < BaseStage
+      def initialize(*args)
+        super(*args)
+
+        @description = "Time between merge request creation and merge/close"
+      end
+
       def median
         @fetcher.calculate_metric(:review,
                                   MergeRequest.arel_table[:created_at],

lib/gitlab/cycle_analytics/staging_stage.rb

 module Gitlab
   module CycleAnalytics
     class StagingStage < BaseStage
+      def initialize(*args)
+        super(*args)
+
+        @description = "From merge request merge until deploy to production"
+      end
+
       def median
         @fetcher.calculate_metric(:staging,
                                   MergeRequest::Metrics.arel_table[:merged_at],

lib/gitlab/cycle_analytics/test_stage.rb

 module Gitlab
   module CycleAnalytics
     class TestStage < BaseStage
+      def initialize(*args)
+        super(*args)
+
+        @description = "Total test time for all commits/merges"
+      end
+
       def median
         @fetcher.calculate_metric(:test,
                                   MergeRequest::Metrics.arel_table[:latest_build_started_at],