Start here. This is the direct spoken answer to practice first.
Why this question matters
Observability is useful only when it helps answer production questions. This drill matters because the production behavior is shaped by platform configuration, code boundaries, and the operational path the team will use during a real incident.
For a production .NET API I would start with requests, response codes, latency, exceptions, dependency calls, structured logs, and availability checks. I would make sure traces include enough context to connect an error to a route, operation, dependency, and deployment version. I would also add a few business metrics where they help answer whether users are actually affected.