fix: Sentry.init register integrations after creating main Hub instead of in the main Hub ctor

[marandaneto]

📢 Type of change

• Bugfix
• New feature
• Enhancement
• Refactoring

📜 Description

fix: Sentry.init register integrations after creating main Hub instead of in the main Hub ctor

💡 Motivation and Context

when integrations are registered on Hub ctor and async integrations are fired,
it might and actually happened that integrations called captureSomething
and the Hub was still NoOp.
Registering integrations Sentry.init(...) makes sure that the Hub is already created.

💚 How did you test it?

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• All tests passing

🔮 Next steps

Find a proper way of registering global and only once integrations.

[bruno-garcia]

happy to merge and ship.
Would be definitely important to have tests for this not to regress

[codecov-commenter]

Codecov Report

Merging #427 into master will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #427      +/-   ##
============================================
- Coverage     60.04%   59.96%   -0.09%     
+ Complexity      813      812       -1     
============================================
  Files            93       93              
  Lines          3742     3742              
  Branches        363      363              
============================================
- Hits           2247     2244       -3     
- Misses         1339     1343       +4     
+ Partials        156      155       -1     

Impacted Files	Coverage Δ	Complexity Δ
sentry-core/src/main/java/io/sentry/core/Hub.java	65.06% <ø> (-0.32%)	59.00 <0.00> (-1.00)
...ntry-core/src/main/java/io/sentry/core/Sentry.java	32.32% <100.00%> (+2.11%)	8.00 <0.00> (+1.00)
...ntry/core/UncaughtExceptionHandlerIntegration.java	71.05% <0.00%> (-3.95%)	9.00% <0.00%> (-1.00%)

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