code: public static void enablePresence(String url, Boolean isCluster, String applicationKey, String privateKey, String channel,
  	      Boolean metadata, Proxy proxy, OnEnablePresence callback) {
  	    Presence.enablePresence(url, isCluster, applicationKey, privateKey, channel, metadata, proxy, callback);
    }
docs: Enables presence for the specified channel with first 100 unique metadata if metadata is set to true (using proxy connection).
  
  <pre>
  Ortc.enablePresence(&quot;http://ortc-developers.realtime.co/server/2.1/&quot;, true, &quot;APPLICATION_KEY&quot;, &quot;PRIVATE_KEY&quot;,
  &quot;CHANNEL&quot;, true, new Proxy(&quot;my.proxy.net&quot;, 1234), new onEnablePresence() {
  
  public void run(Exception error, String result) {
  if (error != null) {
  System.out.println(error.getMessage());
  } else {
  System.out.println(result);
  
  }
  }
  });
  </pre>
  
  @param url
  Server containing the presence service.
  @param isCluster
  Specifies if url is cluster.
  @param applicationKey
  Application key with access to presence service.
  @param privateKey
  The private key provided when the ORTC service is purchased.
  @param channel
  Channel with presence data active.
  @param metadata
  Defines if to collect first 100 unique metadata.
  @param proxy  Object with definition of proxy connection (ibt.ortc.api.Proxy)
  @param callback
  Callback with error and result.