Haproxy in VyOS is built with Prometheus support, and we can consider adding it to the CLI. More details on how to enable it here: https://github.com/haproxy/haproxy/blob/f6373a6ca841ea579b494cfd0b737956358b9ed5/addons/promex/README

From special requirements, I only see that a path to the Prometheus service must be configurable, not to overlap with any real service users may have.

For example, we can introduce a special preconfigured backend for this:

set load-balancing haproxy service prometheus_demo rule 100 url-path exact '/mymetrics'
set load-balancing haproxy service prometheus_demo rule 100 set backend 'prometheus-exporter'

Or just a new node:

set load-balancing haproxy service prometheus_demo rule 100 url-path exact '/mymetrics'
set load-balancing haproxy service prometheus_demo rule 100 prometheus-exporter