Add WithCustomSampler trace option

[arugal]

#84

[codecov-io]

Codecov Report

Merging #85 (7d4bce4) into master (b266e96) will decrease coverage by 0.23%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #85      +/-   ##
==========================================
- Coverage   67.26%   67.02%   -0.24%     
==========================================
  Files          13       13              
  Lines         559      561       +2     
==========================================
  Hits          376      376              
- Misses        149      151       +2     
  Partials       34       34              

Impacted Files	Coverage Δ
trace_opts.go	26.66% <0.00%> (-4.11%)	⬇️

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