Bring diagnostics_tools.md from debug project to docs

doc/administration/index.md

@@ -185,3 +185,5 @@ Learn how to install, configure, update, and maintain your GitLab instance.
 - [Debugging tips](troubleshooting/debug.md): Tips to debug problems when things go wrong
 - [Log system](logs.md): Where to look for logs.
 - [Sidekiq Troubleshooting](troubleshooting/sidekiq.md): Debug when Sidekiq appears hung and is not processing jobs.
+- Useful [diagnostics tools](troubleshooting/diagnostics_tools.md) that are sometimes used by the GitLab
+  Support team.

doc/administration/troubleshooting/diagnostics_tools.md

+---
+type: reference
+---
+
+# Diagnostics tools
+
+These are some of the diagnostics tools the GitLab Support team uses during troubleshooting.
+They are listed here for transparency, and they may be useful for users with experience
+with troubleshooting GitLab. If you are currently having an issue with GitLab, you
+may want to check your [support options](https://about.gitlab.com/support/) first,
+before attempting to use these tools.
+
+## gitlabsos
+
+The [gitlabsos](https://gitlab.com/gitlab-com/support/toolbox/gitlabsos/) utility
+provides a unified method of gathering info and logs from GitLab and the system it's
+running on.
+
+## strace-parser
+
+[strace-parser](https://gitlab.com/wchandler/strace-parser) is a small tool to analyze
+and summarize raw strace data.
+
+## Pritaly
+
+[Pritaly](https://gitlab.com/wchandler/pritaly) takes Gitaly logs and colorizes output
+or converts the logs to JSON.