fix(instrumentation-dataloader): Patch batchLoadFn without creating an instance

[onurtemizkan]

Related: getsentry/sentry-javascript#13869

Which problem is this PR solving?

• The current version of the instrumentation creates an instance (and returns it) in order to patch the DataLoader constructor for batchLoadFn. This breaks the usage where the DataLoader class is extended.

Short description of the changes

• This implementation uses the first argument of the DataLoader constructor (which is batchLoadFn) to patch to avoid creating an instance.

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.84%. Comparing base (9a20e15) to head (60d8499).

Files with missing lines	Patch %	Lines
.../instrumentation-dataloader/src/instrumentation.ts	92.85%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2498      +/-   ##
==========================================
- Coverage   90.85%   90.84%   -0.02%     
==========================================
  Files         159      159              
  Lines        7853     7854       +1     
  Branches     1622     1622              
==========================================
  Hits         7135     7135              
- Misses        718      719       +1     

Files with missing lines	Coverage Δ
.../instrumentation-dataloader/src/instrumentation.ts	98.07% <92.85%> (-0.96%)	⬇️