Add common Praefect DB load to docs

There are two common reasons for the Praefect DB to experience high CPU
load, but currently we don't have these documented.

doc/administration/gitaly/troubleshooting.md

@@ -435,6 +435,15 @@ Here are common errors and potential causes:
     - Praefect cannot reach one or more of its child Gitaly nodes. Try running
       the Praefect connection checker to diagnose.
 
+### Praefect database experiencing high CPU load
+
+Some common reasons for the Praefect database to experience elevated CPU usage include:
+
+- Prometheus metrics scrapes [running an expensive query](https://gitlab.com/gitlab-org/gitaly/-/issues/3796). If you have GitLab 14.2
+  or above, set `praefect['separate_database_metrics'] = true` in `gitlab.rb`.
+- [Read distribution caching](praefect.md#reads-distribution-caching) is disabled, increasing the number of queries made to the
+  database when user traffic is high. Ensure read distribution caching is enabled.
+
 ### Determine primary Gitaly node
 
 To determine the primary node of a repository: