Expose the transferred number of bytes

From Slack discussions.

It would be handy to have access to a `getTransferredBytes()` (or similar) method that would expose the total number of bytes transferred in the last call.

```php
$redis->ping();
$redis->getTransferredBytes(); // 6 bytes

$redis->hgetall('foo');
$redis->getTransferredBytes(); // 1337 bytes
```