fix: set delegate after successful registration

[Flarna]

Set the new TracerProvider as delegate in ProxyTracerProvider only if global registration was successful.

fixes: #104

[codecov]

Codecov Report

Merging #105 (82142be) into main (7ca5a00) will increase coverage by 0.19%.
The diff coverage is 100.00%.

❗ Current head 82142be differs from pull request most recent head a7c96b0. Consider uploading reports for the commit a7c96b0 to get more accurate results

@@            Coverage Diff             @@
##             main     #105      +/-   ##
==========================================
+ Coverage   94.90%   95.09%   +0.19%     
==========================================
  Files          42       42              
  Lines         569      571       +2     
  Branches       89       90       +1     
==========================================
+ Hits          540      543       +3     
+ Misses         29       28       -1     

Impacted Files	Coverage Δ
src/api/trace.ts	100.00% <100.00%> (+3.22%)	⬆️

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[obecny]

lgtm