Implementing namespaces in plugin template

The problem is with your Autoloader set-up. You need to convert your Camelcase namespacese to dashes for locating files as per your current folder structure.

I have added convert function and updated your autoloader function.

In wp-plugin-template/autoloader.php:

<?php
spl_autoload_register( 'autoload_function' );
function autoload_function( $classname ) {
    $class = str_replace( '\\', DIRECTORY_SEPARATOR, str_replace( '_', '-', strtolower(convert($classname)) ) );
    # Create the actual file-path
    $path = WP_PLUGIN_DIR . DIRECTORY_SEPARATOR . $class . '.php';
    # Check if the file exists
    if ( file_exists( $path ) ) {
        # Require once on the file
        require_once $path;
    }
}

function convert($class){
    $class = preg_replace('#([A-Z\d]+)([A-Z][a-z])#','\1_\2', $class);
    $class = preg_replace('#([a-z\d])([A-Z])#', '\1_\2', $class);
    return $class;
}

Also you have to update your top level namespace to WpPluginTemplate in both wp-plugin-template.php and admin/plugin-meta.php as you are using wp-plugin-template as your plugin folder name.

UPDATE:

When autoloader tries to find Plugin_Meta class, it will look into YOUR_PLUGIN_DIR/wp-plugin-template/admin/plugin-meta.php

In wp-plugin-template.php

<?php
/**
 * Plugin Name: WP Plugin Template
 */
# If accessed directly, exit
if ( ! defined( 'ABSPATH' ) ) exit;
# Call the autoloader
require_once( 'autoloader.php' );

use WpPluginTemplate\admin\Plugin_Meta;
new Plugin_Meta;

In wp-plugin-template/admin/plugin-meta.php

<?php
namespace WpPluginTemplate\admin;
class Plugin_Meta {
    public function __construct() {
        add_action( 'plugins_loaded', array($this, 'test' ) );
    }

    public function test() {
        echo 'It works!';
    }
}