[core] Create function to retrieve frame busy time (non-sleeping time)

[bmbkr]

No description provided.

[bmbkr]

I still need to update documentation throughout the repo to include this new function.

That being said, I'm eagerly awaiting your thoughts on this new functionality @raysan5

[raysan5]

@bmbkr I prefer to avoid exposing this internal state, most users probably do not need it and advance users can implement it for their needs or just enable SUPPORT_CUSTOM_FRAME_CONTROL on config.h to have full control over the frame.

Exposing this state would imply exposing other internal states that I think most users do not need at the moment.

[bmbkr]

@raysan5 Understood. Perhaps this can be exposed at some point in the future as part of a metrics interface, simply as a ratio of busy to sleep time. Would be particularly useful for performance measurement, although as you said you can just recompile with custom frame control.

[raysan5]

@bmbkr there was a new feature in that line in the past (#1295) but it required to many changes and coupling and it was discarded...