Display numbers for the "Issue (Tracker)" cycle analytics metric.

1. Code is messy and untested, but it is a start.

app/controllers/projects/cycle_analytics_controller.rb

+class Projects::CycleAnalyticsController < Projects::ApplicationController
+  def show
+    @metrics = {
+      issue: issue
+    }
+  end
+
+  private
+
+  def issue
+    query = <<-HEREDOC
+       WITH ordered_data AS (
+         SELECT extract(milliseconds FROM (COALESCE(first_associated_with_milestone_at, first_added_to_board_at) - issues.created_at)) AS data_point,
+                row_number() over (order by (COALESCE(first_associated_with_milestone_at, first_added_to_board_at) - issues.created_at)) as row_id
+         FROM issues
+         INNER JOIN issue_metrics ON issue_metrics.issue_id = issues.id
+         WHERE COALESCE(first_associated_with_milestone_at, first_added_to_board_at) IS NOT NULL
+       ),
+
+       ct AS (
+         SELECT count(1) AS ct
+         FROM ordered_data
+       )
+
+       SELECT avg(data_point) AS median
+       FROM ordered_data
+       WHERE row_id between (select ct from ct)/2.0 and (select ct from ct)/2.0 + 1;
+    HEREDOC
+
+    ActiveRecord::Base.connection.execute(query).to_a.first['median']
+  end
+end

app/views/projects/cycle_analytics/show.html.haml

+%pre= @metrics

config/routes.rb

@@ -779,6 +779,8 @@ Rails.application.routes.draw do
         resources :environments
+        resource :cycle_analytics
+
         resources :builds, only: [:index, :show], constraints: { id: /\d+/ } do
           collection do
             post :cancel_all