test(bench): use Writer in OTEL encode

[ernado]

name           old time/op    new time/op    delta
OTEL_Write-32     389ns ± 3%     236ns ± 2%  -39.24%  (p=0.000 n=10+10)

name           old speed      new speed      delta
OTEL_Write-32  1.09GB/s ± 3%  1.79GB/s ± 2%  +64.56%  (p=0.000 n=10+10)

[codecov]

Codecov Report

Merging #11 (4897c44) into main (0a14eba) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #11      +/-   ##
==========================================
+ Coverage   99.53%   99.63%   +0.10%     
==========================================
  Files          29       29              
  Lines        1919     1921       +2     
==========================================
+ Hits         1910     1914       +4     
+ Misses          9        7       -2     

Impacted Files	Coverage Δ
w.go	95.55% <100.00%> (+4.85%)	⬆️

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