That's what the Prometheus push gateway is for. While prom itself is poll-based, you can extract data from servers behind a firewall by pushing metrics from an agent into the gateway. No crazy issues.
You posted the same two problems in two different areas of the thread. As mentioned here (https://news.ycombinator.com/item?id=6763780), your first assertion is incorrect. Page loads do take longer, but you also win being able to support multiple frontends (consumer facing web, backend management web, mobile apps, desktop apps, other services, etc) much easier than if you had to maintain server-side rendering as well as an API. After the first page load, however, you're transferring far less data and will be loading pages much quicker. Sure, if you have a mostly static site, it's not worthwile, but if your app is highly personalized, this is a great direction to go.