Change SetFunction signature to return the function's ID, not update

[dmathieu]

Part of #14016

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.68%. Comparing base (c854264) to head (96a279c).
⚠️ Report is 25 commits behind head on main.

Files with missing lines	Patch %	Lines
pdata/pprofile/functions.go	60.00%	2 Missing ⚠️

❌ Your patch status has failed because the patch coverage (60.00%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14032      +/-   ##
==========================================
+ Coverage   91.67%   91.68%   +0.01%     
==========================================
  Files         654      654              
  Lines       42619    42607      -12     
==========================================
- Hits        39069    39065       -4     
+ Misses       2738     2732       -6     
+ Partials      812      810       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[codeboten]

@dmathieu when do you want to merge these changes in? Does it make sense to merge this along with #14030 and #14031 to only need to update contrib once? (i assume there's changes needed over there as a result here)

[dmathieu]

None of those methods are currently used in contrib. So there shouldn't be any change needed there (as proof, contrib tests pass on all 3 PRs).