Add content type header for metrics endpoint

Set the Content-Type header to 'text/plain; version=0.0.4' for the '/metrics' endpoint and CLI requests.

index.php

@@ -3,6 +3,7 @@ require 'vendor/autoload.php';
 require 'config.php';
 
 if($_SERVER["REQUEST_URI"] == '/metrics' || php_sapi_name() == 'cli') {
+    header('Content-Type: text/plain; version=0.0.4');
     foreach ($config as $key => &$c) {
         if(isset($c['command'])) {
             $output=null;