To enable a network connection as a metered connection, open the Charms Bar by pressing the Windows key and I key (
+ 
) on your keyboard. At the bottom of the bar, click Settings, and click the wireless network icon on the bottom:
You should see a list of available wireless networks. Right click on the network you wish to make "metered".
On the menu that appears, choose Set as metered connection.
Windows 8 has a default setting that does not allowed device downloads and Windows updates to occur over metered connections. This setting is controlled via the Devices category of the PC Settings screen. When you go into the Devices settings and scroll down you will see an option for allowing downloads to occur over a metered connection.
By default, this is set to Off, which does not allow downloads to occur. If you wish to allow downloads over a metered connection, you should set this setting to On. It is advised that you keep this setting on Off to help avoid overage charges on your data plans. An example of how Windows Update works when this setting is off and you are on a metered connection can be see in the image below.
As you can see from the above screen, now that metered connections are enabled Windows will not install updates until you are connected to a connection that is unmetered. This will help to protect you from going over any bandwidth limits that you connection may have.