At small scale it's fine. Its architecture totally forecloses any possibility of distributed evaluation, so there is a brick wall of scalability. Its human-readable-text-over-HTTP scraping scheme is also hilariously inefficient. It is not, in short, "planet scale".
Well, to be fair, they are offering Prometheus as a service, and they built it as a façade upon Monarch, which pretty convincingly demonstrates the superiority of Monarch.
I just find a kind of perverse humor in the fact that outside google Prometheus is viewed as alien technology - it even has that condescending name - but inside Google borgmon was considered the punch line of memes and/or a hazing ritual.
IIRC, a core point in early days of Prometheus was a greatly simplified system that built on some of the same ideas, but without the parts that made it into punch line of memes or a hazing ritual for new guys.
While the text based metrics aren't best, they are a reasonable compromise, and I found that it provides a good enough common interface where if needed I can change what collects from it later on.
Are there better options? SURE! I can, however, usually get my 80% of the problem solved with Prometheus integration and worry about scaling later.
https://www.vldb.org/pvldb/vol13/p3181-adams.pdf