Request ability to inject an event marker in tlog realtime

I would like to be able to insert an event marker into the tlog during a flight while recoding. This would be very useful during tuning sessions. For example if I am about to attempt a maneuver, I could press a button to mark the time just before it begins. This could also be useful if something peculiar occurs during the flight and you want to reference it later.
It would be very handy to be able to label these markers, but even without labels it would be very useful.

The issue names no files, tests, or entry points. Start by locating realtime tlog recording and the control used during flight, then determine how an event marker could be inserted and optionally labeled; done means a marker can be added during recording and referenced in the resulting tlog.

Written by the indexing model from the issue text.