How not to show last bracket in a monolog log line?

That's the extra data. The default format of the LineFormatter is "[%datetime%] %channel%.%level_name%: %message% %context% %extra%\n". the username/age is the context, and extra that is typically empty results in this empty array [].

If you use processors to attach data to log records they typically write it to the extra key to avoid conflicts with context info. If it really is an issue for you you can change the default format and omit %extra%.

Edit: As of Monolog 1.11 the LineFormatter has a $ignoreEmptyContextAndExtra parameter in the constructor that lets you remove these, so you can use this:

// the last "true" here tells it to remove empty []'s
$formatter = new LineFormatter(null, null, false, true);
$handler->setFormatter($formatter);

Old question, but throwing out another simple option:

$slackHandler = new \Monolog\Handler\SlackWebhookHandler(...);
$slackHandler->getFormatter()->ignoreEmptyContextAndExtra(true);