feat(dynamic-sampling): Use new span count endpoint

[ArthurKnaus]

Use the newly created endpoint on the frontend.

Closes https://github.com/getsentry/projects/issues/353

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...s/dynamicSampling/utils/useProjectSampleCounts.tsx	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #80732   +/-   ##
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  Coverage   78.39%   78.39%           
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  Files        7209     7209           
  Lines      319305   319304    -1     
  Branches    43976    43976           
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+ Hits       250308   250311    +3     
+ Misses      62622    62613    -9     
- Partials     6375     6380    +5