code: public MetadataBuilder addTracker(String url) {
      initFirstTier();
      announceList.get(announceList.size() - 1).add(url);
      return this;
    }
docs: Multi-tracker Metadata Extension. Add new tracker URL to current tier.
  This method will create first tier automatically if it doesn't exist
  You can find more information about this extension in documentation
  <a href="http://bittorrent.org/beps/bep_0012.html">http://bittorrent.org/beps/bep_0012.html</a>
  
  @param url tracker url