add config option to add only debug! (and not trace!) calls

[guswynn]

in #76588, @jyn514 mentioned that one way we could possibly turn on logging by default is to separate debug! and trace! calls

even if we don't go with it as default, its still nice to separate them so we can have granularity if we want it, AND so we can actually test the perf lost by making it default

r? @jyn514@Mark-Simulacrum

[guswynn]

@rustbot modify labels +A-rustbuild

[jyn514]

I'm not sure who the expected audience for this is. Very few parts of the compiler distinguish between trace! and debug!, so this doesn't have much of a performance impact, and personally I would still keep trace level logging on even if you added this option.

The idea I was thinking of was to move some logging from debug! to trace!. However, that will require building a consensus with the relevant parts of the compiler team that this is a useful thing to do. If you're interested in that route, take a look at the zulip for t-compiler, especially 'improving bootstrap times for contributor fun and profit'.

As-is, though, I don't see much point to this.

[guswynn]

Ill read more into this!

[jyn514]

See also tokio-rs/tracing#975, which has some more discussion.