feat: include otel as custom sampling contex

[saviogl]

Sentry's OpenTelemetry (OTel) adapter does not do the greatest job in adapting the OTel span to a semantically correct sentry span, nor does it expose the OTel span in the context for sampling decisions. This PR adds a OTel span as an additional custom sampling context so it can be used for sampling decisions. This gives clients more opportunities to use the entire context of the OTel span for sampling decisions, and allows replicating sampling mechanisms as originally suggested by the Sentry team via the "Sampling Function".

https://docs.sentry.io/platforms/ruby/configuration/sampling/#setting-a-sampling-function

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---

closes #2684
closes RUBY-86

[solnic]

Thanks for the PR - please rebase this first which should fix the build 😄

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.33%. Comparing base (611ef73) to head (7c8ab68).
⚠️ Report is 43 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2683      +/-   ##
==========================================
+ Coverage   97.31%   97.33%   +0.01%     
==========================================
  Files         144      144              
  Lines        5657     5657              
==========================================
+ Hits         5505     5506       +1     
+ Misses        152      151       -1     

Components	Coverage Δ
sentry-ruby	97.78% <ø> (ø)
sentry-rails	95.72% <ø> (+0.10%)	⬆️
sentry-sidekiq	96.57% <ø> (ø)
sentry-resque	94.44% <ø> (ø)
sentry-delayed_job	94.68% <ø> (ø)
sentry-opentelemetry	99.31% <ø> (ø)

Files with missing lines	Coverage Δ
...lemetry/lib/sentry/opentelemetry/span_processor.rb	100.00% <ø> (ø)

... and 1 file with indirect coverage changes

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