ablity for PrintDiagnosticsPlugin to be toggled via a Resource

[mattdm]

What problem does this solve or what need does it fill?

It's handy to be able to have this simple diagnostic functionality at hand, but it also clutters up the console.

It would be nice to be able to easily turn it on and off (connected with a command-line flag to the program, an environment variable, or some system which changes the state.)

What solution would you like?

A PrintDiagnosticsFlag resource which could be toggled on or off; the system would exit if off.

What alternative(s) have you considered?

One can put the diagnostics in a stage with a run criteria. This adds a lot of boilerplate, though.

Additional context

Not urgent, just would be handy.

[alice-i-cecile]

You should be able to do this with run criteria quite nicely following the addition of system sets in #1144: they're no longer coupled to stages 🎉

That said, showing how to do so might be a nice extension to the frame time diagnostics example.

[mockersf]

a ressource for the diagnostic plugin would make sense, as it's how we want to push plugin configurations

[theotherphil]

This now has a dedicated example enabling_disabling_diagnostic.

https://github.com/bevyengine/bevy/blob/latest/examples/diagnostics/enabling_disabling_diagnostic.rs