Allow modules to extend SearchApiFacetapiAdapter class

Add similar issue in apachesolr.

Thanks for reporting this problem!
However, I’m not 100% sure about your solution. It might be what you want, but in general it doesn’t seem guaranteed that someone implementing their own adapter would still want us to provide the facet info. Also, at this point, I’m extremely hesitant to change any behavior in my Drupal 7 code, especially to add features.
But feel free to either just keep this patch applied to your code, or use a workaround like this:

/**
 * Implements hook_facetapi_facet_info().
 */
function my_module_facetapi_facetapi_facet_info(array $searcher_info) {
  // Masquerade our adapter as the "search_api" adapter.
  if ($searcher_info['adapter'] === 'my_adapter') {
    $searcher_info['adapter'] = 'search_api';
    return search_api_facetapi_facetapi_facet_info($searcher_info);
  }
  return array();
}

Still, leaving open in case this is a more popular request than I thought.