Count users online with Sitecore OMS

When you have a web application, you might want to know how many users are currently online. If you have visited some of web online forums you can see (usually on the first page) the number of currently online users.
This information might be also used to postpone some website jobs if there are too many users online, etc.
How do we know / count how many sessions or users are currently connected to our website in Sitecore?