Fix sum by name in custom metrics

[gabotechs]

Which issue does this PR close?

• Closes #.

Rationale for this change

Something small I spotted. MetricValue::sum_by_name seems to be ignoring custom metrics, while these metrics also have a valid name.

What changes are included in this PR?

The fix and one unit test.

Are these changes tested?

Yes

Are there any user-facing changes?

People can now call sum_by_name in custom metrics

[codecov-commenter]

Codecov Report

❌ Patch coverage is 78.37838% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.44%. Comparing base (1064661) to head (c50f325).

Files with missing lines	Patch %	Lines
datafusion/physical-expr-common/src/metrics/mod.rs	78.37%	8 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #24673   +/-   ##
=======================================
  Coverage   81.44%   81.44%           
=======================================
  Files        1118     1118           
  Lines      399602   399638   +36     
  Branches   399602   399638   +36     
=======================================
+ Hits       325458   325505   +47     
+ Misses      55146    55136   -10     
+ Partials    18998    18997    -1     

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