yii\elasticsearch\ElasticsearchTarget::prepareMessage PHP Method

    public function prepareMessage($message)
    {
        list($text, $level, $category, $timestamp) = $message;
        $result = ['category' => $category, 'level' => Logger::getLevelName($level), '@timestamp' => date('c', $timestamp)];
        if (isset($message[4])) {
            $result['trace'] = $message[4];
        }
        //Exceptions get parsed into an array, text and arrays are passed as is, other types are var_dumped
        if ($text instanceof \Exception) {
            //convert exception to array for easier analysis
            $result['message'] = ['message' => $text->getMessage(), 'file' => $text->getFile(), 'line' => $text->getLine(), 'trace' => $text->getTraceAsString()];
        } elseif (is_array($text) || is_string($text)) {
            $result['message'] = $text;
        } else {
            $result['message'] = VarDumper::export($text);
        }
        if ($this->includeContext) {
            $result['context'] = $this->getContextMessage();
        }
        $message = implode("\n", [Json::encode(['index' => new \stdClass()]), Json::encode($result)]);
        return $message;
    }