Measure usage of Integrated Load Testing through usage ping

Problem

Customers can generate Load Performance Testing reports in CI, but we don't know how often they actually look at them.

Proposal

Add a tracking event that fires when people expand the load performance testing merge request widget.

The counters to add to known_events.yml are:

• load_performance_widget_total - add the user_id to the counter when the user (regardless of plan) opens the load performance testing widget on an MR.

Details for known_events.yml

name: i_testing_load_performance_widget_total
  category: testing
  redis_slot: testing
  aggregation: weekly

This will apply to both Self Managed and gitlab.com users for tracking on the dashboard below.

Redis HLL Counters docs

Success Criteria

Paid GMAU for Load Performance testing usage (how many unique users expanded the widget), displays on the Testing Group Metrics Sisense dashboard.