We should add a log message, at log level 0, whenever the server's profiling settings are changed at run-time. The typical way you change these settings is via the setProfilingLevel shell helper: https://docs.mongodb.com/manual/reference/method/db.setProfilingLevel/index.html
This will make it easier to understand log files. The server logs slow queries to the log file, using the profiling setting to decide what 'slow' means. For someone digging through a log file, and seeing the number of slow queries suddenly go up or down, it might be helpful to know whether that's because the workload changed or the profiling settings changed.