You'd be better off with a modern bike computer that has power meter support, especially if you care about performing intervals. There probably are some free phone apps that track intervals but they would be somewhat obscure compared to fully featured, dedicated bike computer options. TrainerRoad has one, but they're a subscription service and I don't know how useful the app is outdoors. I do know that without ANT+ support on the phone, it and most other apps could only double the power of one of my PowerTap pedals and not get true dual-sided data. That was a huge downside for me.
For simple recording there's free apps like the Wahoo Fitness app, which can sync to lots of different third-party sites like TrainingPeaks and Strava. Golden Cheetah is an analysis program for PCs that can download rides from Strava, but you can't use it when riding outside. Strava used to be able to record rides with sensor support, they ripped out all of it before backtracking and restoring part of the functionality since, but I don't recall the app ever understanding zones or intervals while recording. Plus it was a terrible battery drain. Most of Strava's training and analysis functionality requires a subscription. When I had a free trial, I found their performance metrics utterly useless or worse, so I don't see any value to paying for that.