it:ad:LaunchDarkly

It's called Darkly due to “ You can safely ship the feature, even though it's unfinished, and it will stay dormant in production. Using this approach, called dark deployment, you can release all your code at the end of each development cycle. You no longer need to maintain code branches across multiple development cycles because a given feature requires more than one cycle to complete.”

Essentially, a commercial service to feature flag and track usage. Better than most.