Add support for defining custom perfomance monitors

[greenfox1505]

Describe the project you are working on:
Job Queue

Describe the problem or limitation you are having in your project:
Monitoring Performance

Describe how this feature / enhancement will help you overcome this problem or limitation:
It seems to me that Godot has a fantastic monitoring system for debugging. FPS or physics times or memory use or physics interactions, Godot has a monitor for so many of these. But only core engine features. If a Godot user wants to monitor something else, he has to create his own monitoring system (usually just a label that updates every frame or so).

Can we expose this monitoring system to the user? Allow developers to add metrics to the performance monitoring system and display those in the debugger.

For me, I would like to see how many jobs are completed in a given frame and how that is impacting or influencing the frame rate. Later I'd like to monitor how many of any given entity I have in a scene. Or some other property of those entities.

Show a mock up screenshots/video or a flow diagram explaining how your proposal will work:

#set value every frame
Performance.set_monitor("jobs_completed",jobs_completed)

#as a callback, like set once in `_ready()` and 
#called by the Performance singleton as needed
Performance.create_monitor("jobs_completed",self,"get_jobs_completed")

I believe the callback version is "better" because it removes ambiguity of what happens when the function is called more than once in a single frame. Does it add? Does it replace? Does it average them? Doesn't matter, it doesn't happen. But I come from a JS background, so the world is callbacks to me.

If this enhancement will not be used often, can it be worked around with a few lines of script?:

This would be a core editor feature and likely cannot be added without modifying the debugger itself.

Is there a reason why this should be core and not an add-on in the asset library?:

I don't think the debugger can be modified this way with the asset library.