ensure_subscribed

# File actionpack/lib/action_dispatch/middleware/server_timing.rb, line 30
      def ensure_subscribed
        @mutex.synchronize do
          # Subscribe to all events, except those beginning with "!"
          # Ideally we would be more selective of what is being measured
          @subscriber ||= ActiveSupport::Notifications.subscribe(/\A[^!]/, self)
        end
      end