Add index to project features for usage ping optimization

- need to add conditions to a new index to allow usage ping
  counter to work, where today it is hitting the failback
  condition.

changelogs/unreleased/208887-optimize-project-counters-projects_with_repositories_enabled.yml

+---
+title: Optimize Project counters with respository enabled counter
+merge_request: 26698
+author:
+type: performance

db/migrate/20200309140540_add_index_on_project_id_and_repository_access_level_to_project_features.rb

+# frozen_string_literal: true
+
+class AddIndexOnProjectIdAndRepositoryAccessLevelToProjectFeatures < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_project_features_on_project_id_ral_20'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :project_features, :project_id, where: 'repository_access_level = 20', name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :project_features, INDEX_NAME
+  end
+end

db/schema.rb

@@ -3265,6 +3265,7 @@ ActiveRecord::Schema.define(version: 2020_03_09_195710) do
     t.integer "forking_access_level"
     t.index ["project_id"], name: "index_project_features_on_project_id", unique: true
     t.index ["project_id"], name: "index_project_features_on_project_id_bal_20", where: "(builds_access_level = 20)"
+    t.index ["project_id"], name: "index_project_features_on_project_id_ral_20", where: "(repository_access_level = 20)"
   end
 
   create_table "project_group_links", id: :serial, force: :cascade do |t|