The key distinction is that this internal metrics data that is just used for monitoring is incidental, and can be thrown away without impacting the computation's result. It's just convenient information that humans might want to have so they can reason about the internal state of the overall system.
Anyway, I think such a tool would be highly useful. Also, "progress" is an often overlooked feature, but users actually appreciate it a lot, and often even require it. And you simply can't easily add progress-computation to a system as an after-thought. You have to "weave" it through the design as you build it.
There's also the fact that in such distributed systems, the entity that consumes the result of the computation is generally not the same entity that consumes the metrics.