code: public void enable() throws IOException {
          // This has to be synchronized or it can collide with the check in the task.
          synchronized (optOutLock) {
              // Check if the server owner has already set opt-out, if not, set it.
              if (isOptOut()) {
                  properties.setProperty("opt-out", "false");
                  properties.store(new FileOutputStream(configurationFile), "http://mcstats.org");
              }
  
              // Enable Task, if it is not running
              if (thread == null) {
                  start();
              }
          }
      }
docs: Enables metrics for the server by setting "opt-out" to false in the config file and starting the metrics task.
  
  @throws java.io.IOException