You can get the username yes. The OS version for mobile though.... I don't think so, you can get it for people using Tableau Desktop but I do not believe you have it for the mobile app...
You can find out more information about the available tables here :
If you would like to have more tables you could submit your idea here Ideas
The more votes you get, the better chances of having this implemented.
Hi, completely agree with the above, the User_agent field is quite easy to use with the split function, it also capture connection that are done through the tableau App vs browser.
An extra note; the http_request table only contains a very limited history (typically 7 days) if you want more history try looking at Configure Tableau to Keep All Request History and Daily Desktop Reporting Stats