Remove experiemental notes from Prometheus docs

doc/administration/monitoring/prometheus/gitlab_metrics.md

 # GitLab Prometheus metrics
 
 >**Note:**
-Available since [Omnibus GitLab 9.3][29118]. Currently experimental. For
+Available since [Omnibus GitLab 9.3][29118]. For
 installations from source you'll have to configure it yourself.
 
 To enable the GitLab Prometheus metrics:
@@ -24,7 +24,7 @@ server, because the embedded server configuration is overwritten once every
 
 ## Metrics available
 
-In this experimental phase, only a few metrics are available:
+The following metrics are available:
 
 | Metric                            | Type      | Since | Description |
 |:--------------------------------- |:--------- |:----- |:----------- |

doc/administration/monitoring/prometheus/index.md

@@ -120,7 +120,7 @@ To disable the monitoring of Kubernetes:
 
 ## GitLab Prometheus metrics
 
-> Introduced as an experimental feature in GitLab 9.3.
+> Introduced in GitLab 9.3.
 
 GitLab monitors its own internal service metrics, and makes them available at the `/-/metrics` endpoint. Unlike other exporters, this endpoint requires authentication as it is available on the same URL and port as user traffic.