Fatal error in DatadogHandler::write()

Ran into a major problem, since \Monolog\LogRecord::$level is a read-only property. We disable this setter for now to provide compatibility. Needs work.

Here's a combined diff.

I guess this is related to 📌 Monolog 3 support Active
@daniel.pernold you run monolog_datadog on monolog v3 - right?

I have released a 3.0 release with monolog 3 support.