Monitoring capabilities are missing from Nginx on purpose. They are not and will never be available for free because there is "NGINX Plus".
This is why I recommend switching to HAProxy.
I'd love to just "switch to X", but there is no X which provides all of the above in one great package: Static file serving, load-balanced proxying (TCP/HTTP), fine-grained caching, automatic Let's Encrypt update, API-based configuration (for dynamic upstreams etc), monitoring. Maybe there shouldn't be such a tool. For all other use-cases I go with nginx since it at least provides decent proxying, caching and static file serving.
X == Apache httpd 2.4
Static file serving? Sure!
Load balanced proxying? mod_proxy_balancer is great!
Fine grained caching? mod_disk_cache is also great
Updating loadbalancer bits via the api?
mod_proxy_balancer supports a balancer-manager endpoint for that to do live updates
monitoring? mod_status + prometheus exporter or
mod_prometheus_status
native LE support? https://github.com/icing/mod_md is going to be rolled into upstream apache