Add Apache mod_unique_id's identifier in WebProcessor

mod_unique_id can add a unique ID to every request and also add it to other logfiles. This makes it easy to correlate lines from different logs belonging to the same request. http://httpd.apache.org/docs/2.2/mod/mod_unique_id.html

src/Monolog/Processor/WebProcessor.php

@@ -54,6 +54,7 @@ class WebProcessor
                 'http_method' => isset($this->serverData['REQUEST_METHOD']) ? $this->serverData['REQUEST_METHOD'] : null,
                 'server'      => isset($this->serverData['SERVER_NAME']) ? $this->serverData['SERVER_NAME'] : null,
                 'referrer'    => isset($this->serverData['HTTP_REFERER']) ? $this->serverData['HTTP_REFERER'] : null,
+                'unique_id'   => isset($this->serverData['UNIQUE_ID']) ? $this->serverData['UNIQUE_ID'] : null,
             )
         );