add reading time in twitter card, so it will show up when unfurl

plugin.php

<?php

/*
  Plugin Name: Reading Time
  Plugin URI: http://wordpress.org/extend/plugins/estimated-post-reading-time/
  Description: Calculates an average required time to complete reading a post.
  Version: 1.4
  Author: Konstantinos Kouratoras
  Author URI: http://www.kouratoras.gr
  Author Email: [email protected]
  License: GPL v2

  Copyright 2012 Konstantinos Kouratoras ([email protected])

  This program is free software; you can redistribute it and/or modify
  it under the terms of the GNU General Public License, version 2, as
  published by the Free Software Foundation.

  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with this program; if not, write to the Free Software
  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */

class EstimatedPostReadingTime {

	/* -------------------------------------------------- */
	/* Constructor
	  /*-------------------------------------------------- */

	public function __construct() {

		//Load localisation files
		add_action("plugins_loaded", array(&$this, "estimate_time_plugin_textdomain"));
		
		//Shortcode
		add_action("init", array(&$this, "estimate_time_shortcode_register"));

		//Filter
		$show_in_posts_pages = get_option("eprt_show_in_posts_pages", "0");
		if ($show_in_posts_pages != "0") {
			add_filter("the_content", array(&$this, "estimate_time_content_filter"));
		}
		
		// add to wp head
		add_action('wp_head',array(&$this,'add_meta_twitter'));
		
		
		//Options Page
		require_once( plugin_dir_path(__FILE__) . "/lib/options.php" );
		$estimatedPostReadingTimeOptions = new EstimatedPostReadingTimeOptions();
		add_action("admin_menu", array(&$estimatedPostReadingTimeOptions, "plugin_add_options"));

		
	}

	function estimate_time_plugin_textdomain() {
		load_plugin_textdomain('estimated-post-reading-time',false,dirname( plugin_basename( __FILE__ ) ) . '/languages');
	}

	function estimate_time_content_filter($content) {

		if($estimate_time_shortcode = $this->estimate_time_shortcode())
		{
			$content = '<p>'.$this->estimate_time_shortcode().'</p>'.$content;
			
		}

		return $content;



	}
	
	// new function to add meta twitter
	function add_meta_twitter(){
		if(is_single()){
			echo '<meta name="twitter:label1" value="Reading time">';
		    echo '<meta name="twitter:data1" value="'.$this->estimate_time_shortcode().'">';
		}
		
	}
	

	function estimate_time_shortcode() {

		$result = false;
		
		$show_in_homepage = get_option("eprt_show_in_homepage", "0");
		$show_in_archive = get_option("eprt_show_in_archive", "0");
		
		if ($show_in_homepage == "0" && (is_home() || is_front_page())) {
			return $result;
		}

		if ($show_in_archive == "0" && is_archive()) {
			return $result;
		}

		$wpm = get_option("eprt_words_per_minute", 250);
		$lowercase = get_option("eprt_lowercase", "0");
		
		if (trim($wpm) == "") {
			$wpm = "250";
		}

		global $post;
		$content = strip_tags($post->post_content);		
		$content_words = str_word_count($content);
		$estimated_minutes = floor($content_words / $wpm);

		if ($estimated_minutes < 1) {
			$result = ($lowercase == '1' ? __("less than a minute", "estimated-post-reading-time") : __("Less than a minute", "estimated-post-reading-time"));
		}
		else if ($estimated_minutes > 60) {
			if ($estimated_minutes > 1440){
				$result = ($lowercase == '1' ? __("more than a day", "estimated-post-reading-time") : __("More than a day", "estimated-post-reading-time"));
			}
			else {
				$result = floor($estimated_minutes / 60) . " " . __("hours", "estimated-post-reading-time");
			}
		}
		else if ($estimated_minutes == 1) {
			$result = $estimated_minutes . " " . __("minute", "estimated-post-reading-time");
		}
		else {
			$result = $estimated_minutes . " " . __("minutes", "estimated-post-reading-time");
		}


		return $result;
	}

	function estimate_time_shortcode_register() {
		add_shortcode("est_time", array(&$this, "estimate_time_shortcode"));
	}
}

new EstimatedPostReadingTime();