If your on a tablet then there is the cancel button to use to quit the command also. It is only temporary while we wait for a tool to do it.

If you want a button, I could show you how to setup a button to launch the CustomDistance Measuring tool from Petr's MoI page that shows up in the sidepane. Then you could have a button that says "measure" in the tool pallette if you want.

I have a custom button that looks like this: