design: log value accessors must not panic

[pellared]

Towards #4914

I also updated the prototype with 147762f

[dashpole]

I'm not entirely sure this is necessary to have in the design doc, but i'm not against it if you think that was unclear.

[pellared]

I'm not entirely sure this is necessary to have in the design doc, but i'm not against it if you think that was unclear.

I think it won't hurt and documents why it is different than e.g. slog's value accessors: https://pkg.go.dev/log/slog#Value.Bool

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (dd3b00f) 82.6% compared to head (5694a90) 82.6%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4948     +/-   ##
=======================================
- Coverage   82.6%   82.6%   -0.1%     
=======================================
  Files        238     238             
  Lines      19036   19036             
=======================================
- Hits       15742   15740      -2     
- Misses      3003    3005      +2     
  Partials     291     291             

see 1 file with indirect coverage changes