Unpopular Opinion

It depends. Health metrics, when used correctly, can be useful. I mean, back before smart devices some of us were taught to monitor our heart rates regularly with our fingers, a clock, and if you need to track this over time, a notebook. A smart watch just makes it a lot easier to do. Tracking stuff for health or habit is not a new idea (Benjamin Franklin, for example, promoted tracking behavior and thought in order to improve oneself) and it is useful. Trying to keep track of detailed information in your brain will not work because our brains just don't do that very well, so we invented writing for that purpose.

That being said, tracking is only useful when you need to do it for a specific purpose, and often only in the short term barring exceptions. I tracked my water intake for a while to figure out how much dehydration was a problem for me, how much water I actually needed to feel more comfortable, and about how to schedule that so I could get the right amount each day without a tracker. I used tools to develop discipline, but which discipline is correct and effective. Otherwise, using subjective data only, we might just engage in self torture (ie, relying on what we believe or want to be the case, not necessarily what is the case). Similarly, I've been adding sensors in my home because it's good to know the temperature, humidity, air quality etc especially when we're curious about whether the devices we use for certain conditions are working, necessary, effective, etc. We made some serious quality of life improvements so far, too, thanks to that data.

The short of it is tracking is like doing research in a lab, but people treat it like a gimmicky toy. They don't have a reason to monitor (whatever metric), they just like the idea of doing so, to feel like they live in science fiction.