fix: correct path to autoloader in metrics/getting-started (open-tele…

…metry#1299)

examples/metrics/getting_started.php

@@ -9,7 +9,7 @@
 use OpenTelemetry\SDK\Metrics\MetricReader\ExportingReader;
 use OpenTelemetry\SDK\Resource\ResourceInfoFactory;
 
-require 'vendor/autoload.php';
+require_once __DIR__ . '/../../vendor/autoload.php';
 
 /**
  * Basic async/observable metrics generation example. This uses the console