Add database event tracking for new models

What does this MR do and why?

Related to #390827 (closed)

We want to add db event tracking to two new models. So far we only had it in one model (analytics stage), so I also redesigned test cases into a shared example group.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Enable snowplow micro
2. Enable the previous feature flag: Feature.enable(:product_intelligence_database_event_tracking)
3. Enable the new feature flag: Feature.enable(:product_intelligence_database_event_tracking_batch2)
4. Trigger the new events:
   • Create/update a merge request
   • Create/change status of a vulnerability [project menu -> Security and Compliance -> Vulnerability report]
5. Check if a new event with database_event_create/database_event_update action and model class category has been registered at localhost:9091/micro/good

DB queries:

Before: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/17377/commands/58240

INSERT INTO merge_request_metrics (merge_request_id, target_project_id, updated_at, created_at)
VALUES (146, 31, NOW(), NOW())
ON CONFLICT (merge_request_id)
DO UPDATE SET
target_project_id = EXCLUDED.target_project_id,
updated_at = NOW()

After: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/17377/commands/58241

INSERT INTO merge_request_metrics (merge_request_id, target_project_id, updated_at, created_at)
VALUES (146, 31, NOW(), NOW())
ON CONFLICT (merge_request_id)
DO UPDATE SET
target_project_id = EXCLUDED.target_project_id,
updated_at = NOW()
RETURNING id, merge_request_id, target_project_id, updated_at, created_at

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.