docs/observability: update preprequisite for metrics scraping
Metrics is only scraped for namespaces that are part of the mesh,
update the doc to reflect this.
Part of
@@ -113,6 +113,7 @@ pushing the listener configuration to the pods for Prometheus to reach:
Metrics scraping can be configured using the `osm metrics` command. By default, OSM **does not** configure metrics scraping for pods in the mesh. Metrics scraping can be enabled or disabled at namespace scope such that pods belonging to configured namespaces can be enabled or disabled for scraping metrics.
For metrics to be scraped, the following prerequisites must be met:
+- The namespace must be a part of the mesh, ie. it must be labeled with the `openservicemesh.io/monitored-by` label with an appropriate mesh name. This can be done using the `osm namespace add` command.
- A running service able to scrap Prometheus endpoints. OSM provides configuration for an [automatic bringup of Prometheus](#automatic-bring-up); alternatively users can [bring their own Prometheus](#byo-bring-your-own).
- The `prometheus_scraping` config key in osm-controller's `osm-config` ConfigMap must be set to `"true"`, which is the default configuration.
