Add Puma exporter doc

doc/administration/monitoring/prometheus/puma_exporter.md

+---
+stage: Monitor
+group: Monitor
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
+# Puma exporter **(FREE SELF)**
+
+You can use the [Puma exporter](https://github.com/sapcc/puma-exporter)
+to measure various Puma metrics.
+
+To enable the Puma exporter:
+
+1. [Enable Prometheus](index.md#configuring-prometheus).
+1. Edit `/etc/gitlab/gitlab.rb` to add (or find and uncomment) the following lines. Make sure
+   `puma['exporter_enabled']` is set to `true`:
+
+   ```ruby
+   puma['exporter_enabled'] = true
+   puma['exporter_address'] = "127.0.0.1"
+   puma['exporter_port'] = 8083
+   ```
+
+1. Save the file and [reconfigure GitLab](../../restart_gitlab.md#omnibus-gitlab-reconfigure)
+   for the changes to take effect.
+
+Prometheus begins collecting performance data from the Puma exporter exposed at `localhost:8083`.