trace: Simplify dynamic tracing level application

[jplatte]

Motivation

I've been looking to refactor the trace layer (really kind of rewrite it, but I wanted to read most of the existing code and ideally transform it piece by piece instead of starting from scratch, which would likely lead to lots of going back and forth as I discover new things that the old code supported that aren't that easy).

As part of that, I found the log_pattern_match macros and thought that there had to be a better way. And I think I found a better way :)

Solution

• Don't duplicate event! invocations for different format_args! strings, instead create a custom type that implements Display and create it on-the-fly to then log it
• Don't duplicate event! invocations for when a field is supposed to be captured vs. not, instead make the field value an Option which has the same effect (not 100% the same I think, the FieldSet for the event will always contain the field now, but the ValueSet will still behave the same which should be the only practically relevant thing)
• With all other differences gotten rid of, create a single macro for logging event!s with a non-const level

[davidpdrsn]

awesome!