Documentation and link to explanation of prometheus_multiproc_dir

 - Change the wording a little bit

app/views/admin/application_settings/_form.html.haml

@@ -315,7 +315,9 @@
   %fieldset
     %legend Metrics - Prometheus
     %p
-      Enable a Prometheus metrics endpoint at `#{metrics_path}` to expose a variety of statistics on the health and performance of GitLab. Additional information on authenticating and connecting to the metrics endpoint is available
+      Enable a Prometheus metrics endpoint at
+      %code= metrics_path
+      to expose a variety of statistics on the health and performance of GitLab. Additional information on authenticating and connecting to the metrics endpoint is available
       = link_to 'here', admin_health_check_path
       \. This setting requires a
       = link_to 'restart', help_page_path('administration/restart_gitlab')
@@ -330,7 +332,10 @@
           - unless Gitlab::Metrics.metrics_folder_present?
             .help-block
               %strong.cred WARNING:
-                Environment variable `prometheus_multiproc_dir` does not exist or is not pointing to a valid directory.
+              Environment variable
+              %code prometheus_multiproc_dir
+              does not exist or is not pointing to a valid directory.
+              = link_to icon('question-circle'), help_page_path('administration/monitoring/prometheus/gitlab_metrics', anchor: 'metrics-shared-directory')
 
   %fieldset
     %legend Profiling - Performance Bar

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -45,6 +45,20 @@ In this experimental phase, only a few metrics are available:
 | redis_ping_latency_seconds        | Gauge     | Round trip time of the redis ping |
 | user_session_logins_total         | Counter   | Counter of how many users have logged in |
 
+## Metrics shared directory
+
+GitLab's Prometheus client requires a directory to store metrics data shared between multi-process services.
+Those files are shared among all instances running under Unicorn server.
+The directory needs to be accessible to all running Unicorn's processes otherwise
+metrics will not function correctly.
+
+For best performance its advisable that this directory will be located in `tmpfs`.
+
+Its location is configured using environment variable `prometheus_multiproc_dir`.
+
+If GitLab is installed using Omnibus and `tmpfs` is available then metrics
+directory will be automatically configured.
+
 [← Back to the main Prometheus page](index.md)
 
 [29118]: https://gitlab.com/gitlab-org/gitlab-ce/issues/29118