Transaction#finish should ignore the parent's sampling decision

[st0012]

Fixes #1712

[codecov-commenter]

Codecov Report

Merging #1716 (db5ecbe) into master (ee5eadc) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1716      +/-   ##
==========================================
+ Coverage   98.41%   98.42%   +0.01%     
==========================================
  Files         140      140              
  Lines        7990     7994       +4     
==========================================
+ Hits         7863     7868       +5     
+ Misses        127      126       -1     

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry/transaction.rb	100.00% <100.00%> (ø)
sentry-ruby/spec/sentry/transaction_spec.rb	100.00% <100.00%> (ø)
sentry-ruby/lib/sentry/breadcrumb.rb	100.00% <0.00%> (+3.70%)	⬆️

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