code: public MetadataBuilder addTrackers(Collection<String> trackers) {
      initFirstTier();
      announceList.get(announceList.size() - 1).addAll(trackers);
      return this;
    }
docs: Multi-tracker Metadata Extension. Add all trackers 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 trackers collections of trackers URLs