Backport of docs: warn against enabling Prometheus metrics if not in use (#26569)

Co-authored-by: Tim Gross <[email protected]>

Author: hc-github-team-nomad-core

website/content/docs/configuration/telemetry.mdx

@@ -149,8 +149,10 @@ telemetry {
 
 These `telemetry` parameters apply to [Prometheus].
 
-- `prometheus_metrics` `(bool: false)` - Specifies whether the agent should
-  make Prometheus formatted metrics available at `/v1/metrics?format=prometheus`.
+- `prometheus_metrics` `(bool: false)` - Specifies whether the agent should make
+  Prometheus formatted metrics available at `/v1/metrics?format=prometheus`.
+  Nomad's Prometheus client retains metrics in memory unless scraped, so you
+  should not enable this field unless you are collecting metrics via Promtheus.
 
 ### `circonus` (Apica)