Draft: proxy: Expose Prometheus metrics

Allow exposing metrics over a Prometheus-targeted endpoint with a CLI flag to configure the listening address (for cases like running outside of Docker)

The core part of this is from #40121 (closed)

PS: A Grafana dashboard sample (based on the one posted in the refered issue): KokaKiwi/snowflake$165)
PPS: I'm a novice in Golang development (i'm more of a Rust person 😄), so i'm open to code improvement suggestions 😃