Appender-Tracing benchmark to pass message as attribute - 10% faster

[cijothomas]

Tracing appender benchmarks were using textual message, which is less performant that putting them as structured key-value pair.
https://docs.rs/tracing/latest/tracing/event/struct.Event.html mentions majority should be using fields, but does not strongly recommend against textual message. We (OTel) should prefer showing examples with fields only and no messages. I put a TODO in the appender to special case "message" to populate body.

Simply removing the textual message and storing the same content in a named field (in benchmark), shows 10% perf gain, which shows how much worse is not using fields. For fields, we current allocate+copy the string, but that is something to be revisited and fixed later, which should further speed this up.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.8%. Comparing base (2bba6b0) to head (b9bb850).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2001   +/-   ##
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  Coverage   74.8%   74.8%           
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  Files        122     122           
  Lines      20678   20686    +8     
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+ Hits       15474   15482    +8     
  Misses      5204    5204           

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