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/lib/activity_tracker.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+class ActivityTracker
+  EXPIRE_AFTER = 90.days.seconds
+
+  class << self
+    def increment(prefix)
+      key = [prefix, current_week].join(':')
+
+      redis.incrby(key, 1)
+      redis.expire(key, EXPIRE_AFTER)
+    end
+
+    def record(prefix, value)
+      key = [prefix, current_week].join(':')
+
+      redis.pfadd(key, value)
+      redis.expire(key, value)
+    end
+
+    private
+
+    def redis
+      Redis.current
+    end
+
+    def current_week
+      Time.zone.today.cweek
+    end
+  end
+end