OpenTelemetry: logging #4232

We'll probably (finally) switch to Microsoft.Extensions.Logging for 7.0 (#2103). While doing that, we should also align with OpenTelemetry logging guidelines.

https://opentelemetry.io/docs/reference/specification/logs/overview/#new-first-party-application-logs
https://www.meziantou.net/monitoring-a-dotnet-application-using-opentelemetry.htm