wp-api-custom-meta.php

<?php 
function ar_register_project_meta() {
    register_api_field(
        'project', 
        'meta',   
        array(
            'get_callback' => 'ar_register_project_meta_get',
        )
    );
}

function ar_register_project_meta_get( $object, $field_name, $request ) {
    return array(
        'location'  => get_post_meta( $object[ 'id' ], 'project_location', true ),
        'thumbnail_image' => get_field('thubmnail_image', $object[ 'id'] )
    );
}

add_action( 'rest_api_init', 'ar_register_project_meta' );

?>