Add more instance stats APIs (#6125)

* Add GET /api/v1/instance/peers API to reveal known domains

* Add GET /api/v1/instance/activity API

* Make new APIs disableable, exclude private statuses from activity stats

* Fix code style issue

* Fix week timestamps

app/controllers/api/v1/instances/activity_controller.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+class Api::V1::Instances::ActivityController < Api::BaseController
+  before_action :require_enabled_api!
+
+  respond_to :json
+
+  def show
+    render_cached_json('api:v1:instances:activity:show', expires_in: 1.day) { activity }
+  end
+
+  private
+
+  def activity
+    weeks = []
+
+    12.times do |i|
+      day     = i.weeks.ago.to_date
+      week_id = day.cweek
+      week    = Date.commercial(day.cwyear, week_id)
+
+      weeks << {
+        week: week.to_time.to_i.to_s,
+        statuses: Redis.current.get("activity:statuses:local:#{week_id}") || 0,
+        logins: Redis.current.pfcount("activity:logins:#{week_id}"),
+        registrations: Redis.current.get("activity:accounts:local:#{week_id}") || 0,
+      }
+    end
+
+    weeks
+  end
+
+  def require_enabled_api!
+    head 404 unless Setting.activity_api_enabled
+  end
+end