Go ahead and create the custom metric. Then, you can go into any of the instance types it collects for and go into Resources find that custom metric and click on settings.
Within settings, go ahead and select custom and uncheck enable data collection and set that as default. This will turn it off by default.
Then navigate to the instance you want to collect on and go into resources, settings, and enable it just for that instance (but not set as default).
This is off the top of my head, so let me know if it's not clear or if screenshots would help.