chore(docs): add information about new opt-in metric

docs/Verify-Reloader-Working.md

@@ -60,3 +60,16 @@ When Reloader is unable to reload, `reloader_reload_executed_total{success="fals
 reloader_reload_executed_total{success="false"} 15
 reloader_reload_executed_total{success="true"} 12
 ```
+
+### Opt-in Granular Metrics
+
+Reloader can also export a metric to show the number of reloads by namespace. This feature is disabled by default, as it can lead to high cardinality in clusters with many namespaces.
+
+The metric will have both `success` and `namespace` as attributes:
+
+```text
+reloader_reload_executed_total{success="false", namespace="some-namespace"} 2
+reloader_reload_executed_total{success="true", namespace="some-namespace"} 1
+```
+
+To opt-in, set the environment variable `METRICS_COUNT_BY_NAMESPACE` to `enabled` or set the Helm value `reloader.enableMetricsByNamespace` to `true`.