If you want to get fancy with changes (and I've seen it done) you have something else capable of controlling that percentage setting that is tied in to your monitoring. Start out low, say 1% of requests hitting the new path. Automatically ramp up over time to full 100%. If you see failures, automatically drop back to 0% until it can be ascertained that the failure didn't come from the new code.
Some considerations: you'll need some sort of storage mechanism for these flags - is that a centralized configuration service for all your services? Maybe just a table in your database? But database / network calls are expensive to be adding to every single time your code executes the path in question - maybe it makes sense for your service to cache these values locally...but then doesn't that lose part of the purpose of 'fast rollbacks'? Maybe instead of a local cache you spin up a redis instance - but what if this goes down? Will all your instances default to the same value? Etc, etc, etc.
I'm not saying this approach is bad, only that it has complexity, and I find I generally can get away without it.