How to add metatag module data to the Search API indexes
I do it like this:
First 

  function HOOK_search_api_alter_callback_info() {
  $callbacks['search_api_alter_add_keywords_field'] = array(
    'name' =&gt; t('Keywords field from metatag module'),
    'description' =&gt; t("Adds Keyword field from metatag module to indexed data."),
    'class' =&gt; 'SearchApiAddKeywordField',
  );
  return $callbacks;
}


Second 

class SearchApiAddKeywordField extends SearchApiAbstractAlterCallback {      
  public function alterItems(array &amp;$items) {
  foreach ($items as $id =&gt; &amp;$item) {
      if (!isset($item-&gt;metatags[$item-&gt;language]['keywords']['value'])) {

        $item-&gt;search_api_keyword = NULL;
        continue;
      }
      $item-&gt;search_api_keyword = strtolower ($item-&gt;metatags[$item-&gt;language]['keywords']['value']);
    }
  }

  public function propertyInfo() {
    return array(
      'search_api_keyword' =&gt; array(
        'label' =&gt; t('Keyword field'),
        'description' =&gt; t('Add keyword field to index.'),
        'type' =&gt; 'text',
      ),
    );
  }

}



After Clear cache go to you index filters and check: Keywords field from metatag module
If Boost settings need to be changed (to give more importance to the field) do it now.
Index Again
Go to the search results view - the view you created to list indexed content.
Assuming that you have fulltext search filter in "Searched fields" select the new "field" Keyword field
Test the things out.