protected function insertMetadata($content, $entryIdentifier, array $tags, $lifetime)
    {
        if (!is_string($content)) {
            throw new InvalidDataTypeException('Given data is of type "' . gettype($content) . '", but a string is expected for string cache.', 1433155737);
        }
        $metadata = array(
            'identifier' => $entryIdentifier,
            'tags' => $tags,
            'lifetime' => $lifetime
        );
        $metadataJson = json_encode($metadata);
        $this->metadata[$entryIdentifier] = $metadata;
        return $metadataJson . self::SEPARATOR . $content;
    }