Inserting custom post meta value using WP-REST API

Well, there is a way to update post meta. You have to add the update_callback when registering rest field. See the example below:

function rest_api_player_meta() {
    register_rest_field('sp_player', 'player_meta', array(
            'get_callback' => 'get_player_meta',
           'update_callback' => 'update_player_meta',
           'schema' => null,
        )
    );
}

function get_player_meta($object) {
    $postId = $object['id'];

    return get_post_meta($postId);
}

function update_player_meta($meta, $post) {
    $postId = $post->ID;

    foreach ($meta as $data) {
        update_post_meta($postId, $data['key'], $data['value']);
    }
}

add_action('rest_api_init', 'rest_api_player_meta');

The meta is stored under player_meta property in your JSON object you send when creating a new post.