Add docs for request profiling

Closes #23239

CHANGELOG

@@ -115,6 +115,7 @@ v 8.13.0 (unreleased)
   - Fixes padding in all clipboard icons that have .btn class
   - Fix a typo in doc/api/labels.md
   - API: all unknown routing will be handled with 404 Not Found
+  - Add docs for request profiling
   - Make guests unable to view MRs on private projects
 v 8.12.7

doc/README.md

@@ -49,6 +49,7 @@
 - [Git LFS configuration](workflow/lfs/lfs_administration.md)
 - [Housekeeping](administration/housekeeping.md) Keep your Git repository tidy and fast.
 - [GitLab Performance Monitoring](administration/monitoring/performance/introduction.md) Configure GitLab and InfluxDB for measuring performance metrics.
+- [Request Profiling](administration/monitoring/performance/request_profiling.md) Get a detailed profile on slow requests.
 - [Monitoring uptime](user/admin_area/monitoring/health_check.md) Check the server status using the health check endpoint.
 - [Debugging Tips](administration/troubleshooting/debug.md) Tips to debug problems when things go wrong
 - [Sidekiq Troubleshooting](administration/troubleshooting/sidekiq.md) Debug when Sidekiq appears hung and is not processing jobs.

doc/administration/monitoring/performance/request_profiling.md

+# Request Profiling
+
+## Procedure
+1. Grab the profiling token from `Monitoring > Requests Profiles` admin page
+(highlighted in a blue in the image below).
+![Profile token](img/request_profiling_token.png)
+1. Pass the header `X-Profile-Token: <token>` to the request you want to profile. You can use any of these tools
+    * [ModHeader](https://chrome.google.com/webstore/detail/modheader/idgpnmonknjnojddfkpgkljpfnnfcklj) Chrome extension
+    * [Modify Headers](https://addons.mozilla.org/en-US/firefox/addon/modify-headers/) Firefox extension
+    * `curl --header 'X-Profile-Token: <token>' https://gitlab.example.com/group/project`
+1. Once request is finished (which will take a little longer than usual), you can
+view the profiling output from `Monitoring > Requests Profiles` admin page.
+![Profiling output](img/request_profile_result.png)
+
+## Cleaning up
+Profiling output will be cleared out every day via a Sidekiq worker.

doc/development/performance.md

@@ -34,10 +34,11 @@ graphs/dashboards.
 ## Tooling
-GitLab provides two built-in tools to aid the process of improving performance:
+GitLab provides built-in tools to aid the process of improving performance:
 * [Sherlock](profiling.md#sherlock)
 * [GitLab Performance Monitoring](../monitoring/performance/monitoring.md)
+* [Request Profiling](../administration/monitoring/performance/request_profiling.md)
 GitLab employees can use GitLab.com's performance monitoring systems located at
 <http://performance.gitlab.net>, this requires you to log in using your