Controlled by environment variables. When enabled, logging is written to standard output that can be further be processed in a spreadsheet. There is practically no impact on the system if not enabled.
The measurements taken are somewhat ad-hoc and most of them require some manual effort to give useful output. Mostly in the form of graphs. Polishing the code to make it more automated is possible. However, the main goal is to help identify performance bottlenecks. Some of the telemetry code can easily become irrelevant as the suspected bottlenecks have been checked or fixed. So there isn't much point in spending a lot of time on this.