Add javascript example on how we could track UI events using UsageData#increment_unique_users API

Add an example how we can track UI events using UsageData#increment_unique_users API

Example of API call

curl --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/increment_unique_users" -d "event=event_name>"

Resources

https://docs.gitlab.com/ee/development/telemetry/snowplow.html

Problem to fix

Add an example of how this API call can be called when a specific action in UI is triggered, for example Expand Code quality button

cc @gitlab-org/growth/telemetry/engineers