Windows 8.1 has buried this setting deep inside PC Settings. The normal method to access it is :
- Press the Win+I keys together and then click 'PC Settings' at the bottom.
- Click or tap the "Network" item on the left side.
- Click or tap your connection's name on the right side - you have to left-click again on the connection name.
- Here you will be able to enable the option "Set as metered connection" and see other details about that connection.
If that does not work for your bluetooth dialup connection, you might need an updated driver version from the vendor of the dial-up modem that implements the new metering support, for Windows to effectively monitor and limit bandwidth consumption.