src/StackdriverExporter.php
protected function getCallback()
    {
        if (!isset(self::$client)) {
            self::$client = new TraceClient($this->clientConfig);
        }

        return [self::$client, $this->batchMethod];
    }
protected function getCallback()
    {
        if (!isset(self::$client)) {
            self::$client = new TraceClient($this->clientConfig);
        }

        return [self::$client, $this->batchMethod];
    }
Returns an array representation of a callback which will be used to write
batch items.

@return array