Fix MergeSplit issue that ignores the initial message size

[bogdandrutu]

This PR simplifies the MergeSplit logic while fixing a bug (see TestMergeSplitManySmall* on how to replicate that). The performance is also slightly improved by reducing some allocations, see:

Before:

goos: darwin
goarch: arm64
pkg: go.opentelemetry.io/collector/exporter/exporterhelper
cpu: Apple M2 Max
BenchmarkSplittingBasedOnItemCountManySmallTraces
BenchmarkSplittingBasedOnItemCountManySmallTraces-12    	     390	   3029960 ns/op	 4850499 B/op	   81094 allocs/op
PASS

After:

goos: darwin
goarch: arm64
pkg: go.opentelemetry.io/collector/exporter/exporterhelper
cpu: Apple M2 Max
BenchmarkSplittingBasedOnItemCountManySmallTraces
BenchmarkSplittingBasedOnItemCountManySmallTraces-12    	     414	   2898061 ns/op	 4850453 B/op	   81092 allocs/op

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.26%. Comparing base (643a35f) to head (9d77119).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12257      +/-   ##
==========================================
- Coverage   91.30%   91.26%   -0.05%     
==========================================
  Files         465      465              
  Lines       25658    25538     -120     
==========================================
- Hits        23426    23306     -120     
  Misses       1816     1816              
  Partials      416      416              

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