Gather data on the adoption and usage of Special:Investigate.
The questions we would like to answer, and how these can be answered by the schema, are outlined in T246951.
The schema is here: https://meta.wikimedia.org/wiki/Schema:SpecialInvestigate
Which actions to log
The schema defines the following actions to be logged:
 Logging the 'end' action is dependent on T256888: Add 'End investigation' button to Special:Investigate. As discussed with @Niharika, there are a number of questions to resolve on that task, so logging that action is not included in this task.
 These are logged from the client, so we'll add this in a separate task: T257031