Additional internal event properties handling

What does this MR do and why?

Related to #434504 (closed)

Make it possible to pass property, value & label to InternalEvents.

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Before	After

How to set up and validate locally

1. Enable snowplow micro
2. Trigger an event that has the new attributes: for example, open the console and perform Gitlab::InternalEvents.track_event('user_viewed_dashboard_list', user: User.first, additional_properties: { label: 'event_label', property: 'event_prop', value: 123 })
3. Check if a new event with that includes the sent property, label and value has been registered at localhost:9090/micro/good
4. Add an event with mapping: following the new instructions in the internal_event_instrumentation/quick_start.md file, add a properties section to a new or an existing event file. For example, add this segment into the user_viewed_dashboard_list.yml file:

properties:
  user_role:
    external_key: label

6. Clean up the memoized events library by running reload! in the console
7. Trigger this event using the mapped keys, for example: Gitlab::InternalEvents.track_event('user_viewed_dashboard_list', user: User.first, additional_properties: { user_role: 'admin', value: 14 })
8. Check if a new event with that includes the sent property, label and/or value has been registered at localhost:9090/micro/good

doc/development/internal_analytics/internal_event_instrumentation/migration.md

@@ -20,7 +20,7 @@ If you are already tracking events in Snowplow, you can also start collecting me
 
 The event triggered by Internal Events has some special properties compared to previously tracking with Snowplow directly:
 
-1. The `label`, `property` and `value` attributes are not used within Internal Events and are always empty.
+1. The `label`, `property` and `value` attributes are not used within frontend Internal Events and are always empty.
 1. The `category` is automatically set to the location where the event happened. For Frontend events it is the page name and for Backend events it is a class name. If the page name or class name is not used, the default value of `"InternalEventTracking"` will be used.
 
 Make sure that you are okay with this change before you migrate and dashboards are changed accordingly.
@@ -37,9 +37,11 @@ Gitlab::Tracking.event(name, 'ci_templates_unique', namespace: namespace,
 The code above can be replaced by this:
 
 ```ruby
-Gitlab::InternalEvents.track_event('ci_templates_unique', namespace: namespace, project: project, user: user)
+Gitlab::InternalEvents.track_event('ci_templates_unique', namespace: namespace, project: project, user: user, additional_properties: { label: label })
 ```
 
+The `label`, `property` and `value` attributes need to be sent inside the `additional_properties` hash. In case they were not included in the original call, the `additional_properties` argument can be skipped.
+
 In addition, you have to create definitions for the metrics that you would like to track.
 
 To generate metric definitions, you can use the generator: