[feature request] Support metrics sources other than Prometheus (e.g. Datadog, or direct Lumina scrape) #142

Veneer currently requires a Prometheus HTTP query API (prometheusUrl).
Orgs that standardize on DataDog and don't run in-cluster Prometheus are blocked from adopting Veneer — even though Lumina itself works with DataDog.

Motivation — Lumina only exposes metrics → Datadog Agent scrapes it, no Prometheus needed.
Veneer queries Prometheus, and Datadog has no PromQL read API → no supported way to point Veneer at it.
So a Datadog shop can run Lumina but not Veneer without standing up Prometheus solely to feed Veneer.

Can we scrape Lumina's /metrics directly ?