Add request env to sampling context

[st0012]

Closes #1732

[codecov-commenter]

Codecov Report

Merging #1749 (a4675ab) into master (d679277) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1749      +/-   ##
==========================================
+ Coverage   98.39%   98.41%   +0.01%     
==========================================
  Files         141      141              
  Lines        8046     8052       +6     
==========================================
+ Hits         7917     7924       +7     
+ Misses        129      128       -1     

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry/rack/capture_exceptions.rb	95.00% <100.00%> (ø)
...y-ruby/spec/sentry/rack/capture_exceptions_spec.rb	100.00% <100.00%> (ø)
sentry-ruby/lib/sentry/breadcrumb.rb	96.29% <0.00%> (-3.71%)	⬇️
sentry-ruby/lib/sentry/background_worker.rb	100.00% <0.00%> (+5.40%)	⬆️

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[st0012]

@sl0thentr0py I think having the entire request env can enable the most flexible sampling logic? Since the sampling context should be tossed away right after the sampling is done, I don't see any obvious issue for it. Do you have any concern on this?

[sl0thentr0py]

@st0012 yeah that's perfect