[chore][exporter][batcher] Improve exporter.request merge splitting performance by caching items count

[sfc-gh-sili]

Description

This PR improves item-based merge splitting by caching item count in exporter.request. Benchmarks shows benefits the case where many requests are merged in the same batch. We observed roughly 30 - 75% latency improvement.

Event Type	Before (ns/merge)	After (ns/merge)	Diff percentage
Logs	3643.971	2513.947	-31%
Metrics	24550.572	5847.417	-76%
Traces	5172.710	3233.694	-37%

Link to tracking issue

Fixes #12137

Testing

This PR adds four benchmarks for each event type

1. 1000 requests are merged into the same batch.
2. every incoming request is slightly above the limit and will cause a split.
3. the incoming request is split into 10 batches

Benchmark 1 is designed to test the performance of merging, while Benchmark 2 and 3 are primarily designed to test the performance of splitting. Benchmarks indicate that the optimization implemented in this PR improves merging performance.

Before:

BenchmarkSplittingBasedOnItemCountManySmallLogs-10                    	     316	   3643971 ns/op
BenchmarkSplittingBasedOnItemCountManyLogsSlightlyAboveLimit-10       	      43	  25643229 ns/op
BenchmarkSplittingBasedOnItemCountHugeLogs-10                         	      39	  27257268 ns/op
BenchmarkSplittingBasedOnItemCountManySmallMetrics-10                 	      43	  24783318 ns/op
BenchmarkSplittingBasedOnItemCountManyMetricsSlightlyAboveLimit-10    	      16	  67046630 ns/op
BenchmarkSplittingBasedOnItemCountHugeMetrics-10                      	      13	  81208615 ns/op
BenchmarkSplittingBasedOnItemCountManySmallTraces-10                  	     235	   5172710 ns/op
BenchmarkSplittingBasedOnItemCountManyTracesSlightlyAboveLimit-10     	      37	  31968305 ns/op
BenchmarkSplittingBasedOnItemCountHugeTraces-10                       	      30	  35512446 ns/op

After:

BenchmarkSplittingBasedOnItemCountManySmallLogs-10                    	     507	   2513947 ns/op
BenchmarkSplittingBasedOnItemCountManyLogsSlightlyAboveLimit-10       	      40	  26553098 ns/op
BenchmarkSplittingBasedOnItemCountHugeLogs-10                         	      38	  27769027 ns/op
BenchmarkSplittingBasedOnItemCountManySmallMetrics-10                 	     189	   5847417 ns/op
BenchmarkSplittingBasedOnItemCountManyMetricsSlightlyAboveLimit-10    	      16	  63132219 ns/op
BenchmarkSplittingBasedOnItemCountHugeMetrics-10                      	      14	  81363741 ns/op
BenchmarkSplittingBasedOnItemCountManySmallTraces-10                  	     367	   3253624 ns/op
BenchmarkSplittingBasedOnItemCountManyTracesSlightlyAboveLimit-10     	      38	  32017175 ns/op
BenchmarkSplittingBasedOnItemCountHugeTraces-10                       	      31	  34387539 ns/op

Documentation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.74%. Comparing base (9757ead) to head (06b8416).
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12136      +/-   ##
==========================================
+ Coverage   91.73%   91.74%   +0.01%     
==========================================
  Files         463      463              
  Lines       24819    24852      +33     
==========================================
+ Hits        22767    22800      +33     
  Misses       1670     1670              
  Partials      382      382              

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