How to add meta field to REST API Response of a WordPress Posts using register_meta ?

WP REST API

You can use two methods for adding the new meta field to Rest API. By using register_meta() function and register_rest_field() function.

1)register_meta() function.
  By using this function you can add additional meta key to API response by adding the 'show_in_rest' => true, parameter to arguments.
  
  eg: 
  $object_type = 'post';
  $args1 = array( // Validate and sanitize the meta value.
    // Note: currently (4.7) one of 'string', 'boolean', 'integer',
    // 'number' must be used as 'type'. The default is 'string'.
    'type'         => 'string',
    
    // Shown in the schema for the meta key.
    'description'  => 'A meta key associated with a string meta value.',
    
    // Return a single value of the type.
    'single'       => true,
    
    // Show in the WP REST API response. Default: false.
    'show_in_rest' => true,
);
register_meta( $object_type, 'meta_key_name', $args1 );

By using this method you can see the meta key added in the API Json reponse. You can check by usnig the URL

http://site_url.com/wp-json/wp/v2/posts/