How to access to retain flag value after received a message with async_receive ?

Hello,

I'm new to this library and to MQTT in general. But I search a lot and don't find any answer to my question.

After receiving a message from my broker I want to check if this one was a retain message or not. I thought that it will exist a manner to access to the retain flag value of the message received but I didn't find anything like this.

Please tell me if I have not be clear enough.

Start by reading the async_receive API and MQTT message documentation to determine whether the retain flag is exposed. The issue does not name a source file or test; done would require documenting the existing access path or clarifying the missing API and its expected behavior.

Written by the indexing model from the issue text.