Change SetMapping signature to return the mapping'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 91.62%. Comparing base (5174026) to head (74856fe).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
pdata/pprofile/mappings.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   #14030      +/-   ##
==========================================
+ Coverage   91.61%   91.62%   +0.01%     
==========================================
  Files         655      655              
  Lines       42790    42778      -12     
==========================================
- Hits        39200    39196       -4     
+ Misses       2766     2760       -6     
+ Partials      824      822       -2     

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[dmathieu]

Note that there is no contrib change required here, since this method isn't used there.