Update Prometheus.php

Author: ohmydevops

src/Log/Prometheus.php

@@ -124,7 +124,7 @@ public function onPhpbuEnd(End $event)
         foreach ($this->backupStats as $backupName => $backupStats) {
             $this->write(
                 'phpbu_backup_size{name="' . $backupName . '"} ' .
-                $this->backupStats[$backupName]['size'] = " " ? 0 : $this->backupStats[$backupName]['size'] .
+                ($this->backupStats[$backupName]['size'] = " " ? "0" : $this->backupStats[$backupName]['size']) .
                 PHP_EOL
             );
         }