Update Prometheus namespace docs

docs/configuration.md

@@ -236,6 +236,7 @@ Check out the [New Relic](new_relic.md) guide to learn more.
 imgproxy can collect its metrics for Prometheus. Specify binding for Prometheus metrics server to activate this feature:
 
 * `IMGPROXY_PROMETHEUS_BIND`: Prometheus metrics server binding. Can't be the same as `IMGPROXY_BIND`. Default: blank.
+* `IMGPROXY_PROMETHEUS_NAMESPACE`: Namespace (prefix) for improxy metrics. Defaulr: blank.
 
 Check out the [Prometheus](prometheus.md) guide to learn more.
 

docs/prometheus.md

@@ -3,8 +3,8 @@
 imgproxy can collect its metrics for Prometheus. To use this feature, do the following:
 
 1. Set `IMGPROXY_PROMETHEUS_BIND` environment variable. Note that you can't bind the main server and Prometheus to the same port;
-2. Set `IMGPROXY_PROMETHEUS_NAMESPACE` to prepend prefix to the names of metrics.
-   I.e. with `IMGPROXY_PROMETHEUS_NAMESPACE=imgproxy` names will look like `imgproxy_requests_total`.  
+2. _(optional)_ Set `IMGPROXY_PROMETHEUS_NAMESPACE` to prepend prefix to the names of metrics.
+   I.e. with `IMGPROXY_PROMETHEUS_NAMESPACE=imgproxy` names will look like `imgproxy_requests_total`.
 3. Collect the metrics from any path on the specified binding.
 
 imgproxy will collect the following metrics: