Merge branch 'document-custom-event-tracking' into 'master'

Document how to track custom events

## What does this MR do?

This documents how one can track custom events.

## Are there points in the code the reviewer needs to double check?

Spelling, etc.

## Why was this MR needed?

This wasn't documented.

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/22070

See merge request !6310

doc/development/instrumentation.md

@@ -137,3 +137,18 @@ end
 ```
 Here the final value of `sleep_real_time` will be `3`, _not_ `1`.
+
+## Tracking Custom Events
+
+Besides instrumenting code GitLab Performance Monitoring also supports tracking
+of custom events. This is primarily intended to be used for tracking business
+metrics such as the number of Git pushes, repository imports, and so on.
+
+To track a custom event simply call `Gitlab::Metrics.add_event` passing it an
+event name and a custom set of (optional) tags. For example:
+
+```ruby
+Gitlab::Metrics.add_event(:user_login, email: current_user.email)
+```
+
+Event names should be verbs such as `push_repository` and `remove_branch`.