ref(grouping): Combine functions gathering metrics into one

[lobsterkatie]

When we had two versions of the grouping code (optimized and non-optimized), the differences in their logic meant that the helper recording metrics had to be split into two. (In the non-optimized path, we didn't have the information we needed for certain metrics early enough to record them in the main metrics-gathering function.) Now that we've switched to only using the optimized path (and have removed the old path), we can pull those straggler metics into the main metrics helper.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@                             Coverage Diff                             @@
##           kmclb-remove-save_aggregate-and-friends   #78262      +/-   ##
===========================================================================
+ Coverage                                    77.18%   78.12%   +0.94%     
===========================================================================
  Files                                         7067     7067              
  Lines                                       311787   311786       -1     
  Branches                                     50957    50957              
===========================================================================
+ Hits                                        240638   243570    +2932     
+ Misses                                       59237    56461    -2776     
+ Partials                                     11912    11755     -157