Add a StatsCounter implementation for Prometheus

[kakkoyun]

I recently implemented it in our downstream project. I'd be happy to contribute back to the upstream if you want to have it. https://github.com/parca-dev/parca-agent/blob/main/pkg/cache/stats_counter.go

I understand if you want to avoid bloating the project. Since Prometheus is ubiquitous enough, maybe you'd like to have it in the repo.