The key is

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\NetworkList\DefaultMediaCost]

There are entries under it for 3G, 4G, Default, Ethernet and Wifi. The normal settings for 3G and 4G are 2. For Ethernet and Wifi (and Default) it is 1. 2 means metered, 1 means not metered. So to make Ethernet Metered, the value needs to be changed from 1 to 2.

The key is protected: to change the values you will need to change its owner to Administrators or your own username.

To make sure that all connections are metered you can set them all to Metered as in this Registry extract:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\NetworkList\DefaultMediaCost]
"3G"=dword:00000002
"4G"=dword:00000002
"Default"=dword:00000002
"Ethernet"=dword:00000002
"WiFi"=dword:00000002