For the server instances, I can't imagine 4 CPUs/8 gigs RAM is really needed 24/7, but can imagine it being needed in bursts, and I assume GCP has various "elastic" auto-scaling products for that. Would love to know if your metrics indicate that this is the smallest box you could comfortably run on, or if it's just a best-guess. Could potentially spin down blue-green environments after some set amount of time (once you know a deploy "succeeded"), as well.
For Metabase, I can't find any "system requirements," but I'm not _super_ shocked by the price tag. I've always liked the idea of hosting my own tools instead of relying on some SaaS's free/trial tiers (particularly for things like analytics, logging, or metrics), but a lot of the open source options out there assume you have a pretty hefty box to throw it at. At the same time, I haven't found any better alternative other than just doing ad-hoc analyzing in SQL (using a GUI like Postico).