Add additional properties to monitor

What does this MR do and why?

It makes additional_properties sent to Internal Events visible in the monitor since it is practical for debugging purposes.

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Before	After

How to set up and validate locally

1. Start the monitor in one terminal: rails runner scripts/internal_events/monitor.rb perform_epics_action.
2. Create a few events in gdk rails console:

   Gitlab::InternalEvents.track_event('perform_epics_action', user: User.first, project: Project.first, additional_properties: {label: 'abc', property: 'created', value: 42})
   Gitlab::InternalEvents.track_event('perform_epics_action', additional_properties: {label: 'foo'})
   Gitlab::InternalEvents.track_event('perform_epics_action')

3. The Snowplow events table in the monitor should now look something like this:

+-------------------------------------------------------------------------------------------------------------------------------------------------+
|                                                                 SNOWPLOW EVENTS                                                                 |
+----------------------+--------------------------+-----------------------+---------+--------------+------------+------+-------+----------+-------+
| Event Name           | Collector Timestamp      | Category              | user_id | namespace_id | project_id | plan | Label | Property | Value |
+----------------------+--------------------------+-----------------------+---------+--------------+------------+------+-------+----------+-------+
| perform_epics_action | 2024-05-02T20:14:52.127Z | InternalEventTracking | 1       | 22           | 1          | free |       |          |       |
| perform_epics_action | 2024-05-02T20:14:47.092Z | InternalEventTracking | 1       | 22           | 1          | free | abc   |          |       |
| perform_epics_action | 2024-05-02T20:13:28.627Z | InternalEventTracking | 1       | 22           | 1          | free | abc   | created  | 42.0  |
+----------------------+--------------------------+-----------------------+---------+--------------+------------+------+-------+----------+-------+

Related to #435338 (closed)