Fix memory leak caused by throwableToSpan

[adinauer]

📜 Description

The Hub holds on to a WeakHashMap called throwableToSpan which maps root causes of exceptions to Spans. The memory leak was caused by the Span holding a strong reference to the throwable which contains the root cause that is also used as the key for the WeakHashMap. This means the value of the WeakHashMap was causing its own key not to be garbage collected and the memory was never freed.

The fix is to weakly hold on to the Span so it can be garbage collected and thus also the key can be garbage collected afterwards.

💡 Motivation and Context

Fixes #2225

💚 How did you test it?

Manual testing using debugger on a modified sample application, then using visualvm to trigger GC manually.

📝 Checklist

• I reviewed the submitted code
• I added tests to verify the changes
• I updated the docs if needed
• No breaking changes

🔮 Next steps

[bruno-garcia]

Makes sense to me. Thanks Alex

[codecov-commenter]

Codecov Report

Base: 80.64% // Head: 80.62% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (b372548) compared to base (d425298).
Patch coverage: 50.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2227      +/-   ##
============================================
- Coverage     80.64%   80.62%   -0.02%     
  Complexity     3357     3357              
============================================
  Files           240      240              
  Lines         12331    12335       +4     
  Branches       1636     1638       +2     
============================================
+ Hits           9944     9945       +1     
- Misses         1781     1783       +2     
- Partials        606      607       +1     

Impacted Files	Coverage Δ
sentry/src/main/java/io/sentry/Hub.java	74.75% <50.00%> (-0.51%)	⬇️

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