Make applicationName mandatory in LogstashFormatter

src/Monolog/Formatter/LogstashFormatter.php

@@ -41,8 +41,13 @@ class LogstashFormatter extends NormalizerFormatter
      */
     protected $contextPrefix;
 
-
-    public function __construct($systemName = null, $applicationName = null, $extraPrefix = null, $contextPrefix = 'ctxt_')
+    /**
+     * @param string $applicationName the application that sends the data, used as the "type" field of logstash
+     * @param string $systemName      the system/machine name, used as the "source" field of logstash, defaults to the hostname of the machine
+     * @param string $extraPrefix     prefix for extra keys inside logstash "fields"
+     * @param string $contextPrefix   prefix for context keys inside logstash "fields", defaults to ctxt_
+     */
+    public function __construct($applicationName, $systemName = null, $extraPrefix = null, $contextPrefix = 'ctxt_')
     {
         //log stash requires a ISO 8601 format date
         parent::__construct('c');