breaking change: use a key type

[muir]

Changes the key type for attributes to be a typed string.
Adds a cache of JSON encodings for the keys.

[codecov-commenter]

Codecov Report

Attention: 57 lines in your changes are missing coverage. Please review.

Comparison is base (a5ad1de) 76.99% compared to head (20b9966) 76.96%.

Files	Patch %	Lines
xopotel/export.go	63.46%	19 Missing ⚠️
xopotel/otel.go	88.59%	13 Missing ⚠️
xopbase/skip.go	0.00%	9 Missing ⚠️
xoppb/replay.go	88.05%	6 Missing and 2 partials ⚠️
xopconsole/replay.go	75.00%	4 Missing ⚠️
xopotel/buffered.go	60.00%	2 Missing ⚠️
xoprecorder/replay.go	95.65%	1 Missing ⚠️
xopresty/resty.go	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #95      +/-   ##
==========================================
- Coverage   76.99%   76.96%   -0.03%     
==========================================
  Files          68       69       +1     
  Lines       11204    11233      +29     
==========================================
+ Hits         8627     8646      +19     
- Misses       2061     2072      +11     
+ Partials      516      515       -1     

Flag	Coverage Δ
go_tests	76.96% <89.20%> (-0.03%)	⬇️

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