Setting json formatter in monolog

Question

How can the logs be set to json format in monolog....

$logger = new StreamHandler(__DIR__.'/my_app.log');
$logger->setFormatter( new JsonFormatter() );
$logger->pushHandler($logger);

This doesn't seems to be correct as it shows error

Call to undefined method Monolog\Logger::setFormatter()

also I would like my log to be recorded like this:

message{
a="something";
b="something else"
}

instead of:

message{a="something";b="something else"}