[processor/batchprocessor] Improve batch processor edge case performance

[asweet-confluent]

Description

As the batch size approaches one, the time complexity of the batch processor approaches O(n^2). This is caused by repeatedly counting the number of data points in a message. This commit introduces a "short-circuit" behavior that allows the core loop to exit quickly when the desired number of data points has been found.

Testing

Performance

I added a new benchmark for splitMetrics that tests performance with various numbers of metrics and a batch size of one. This is the before and after comparison produced by benchstat:

goos: linux
goarch: amd64
pkg: go.opentelemetry.io/collector/processor/batchprocessor
cpu: 11th Gen Intel(R) Core(TM) i9-11950H @ 2.60GHz
                             │   old.txt    │               new.txt               │
                             │    sec/op    │   sec/op     vs base                │
TraceSizeSpanCount-16          3.326n ± 13%   3.181n ± 5%   -4.37% (p=0.011 n=20)
BatchMetricProcessor-16        366.9µ ±  8%   416.7µ ± 7%  +13.58% (p=0.003 n=20)
MultiBatchMetricProcessor-16   419.6µ ±  6%   404.8µ ± 3%   -3.54% (p=0.021 n=20)
SplitMetrics1k-16              4.063m ±  7%   1.264m ± 4%  -68.88% (p=0.000 n=20)
SplitMetrics16k-16             841.3m ±  3%   189.4m ± 2%  -77.48% (p=0.000 n=20)
SplitMetrics64k-16             17.599 ±  5%    3.203 ± 9%  -81.80% (p=0.000 n=20)
geomean                        1.771m         862.5µ       -51.29%

Correctness

I'm relying on the existing test suite to catch issues here.

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 8 lines in your changes missing coverage. Please review.

Project coverage is 91.56%. Comparing base (a33fdf3) to head (051a987).

Files with missing lines	Patch %	Lines
processor/batchprocessor/splitlogs.go	88.00%	2 Missing and 1 partial ⚠️
processor/batchprocessor/splittraces.go	87.50%	2 Missing and 1 partial ⚠️
processor/batchprocessor/splitmetrics.go	95.74%	1 Missing and 1 partial ⚠️

❌ Your patch check has failed because the patch coverage (91.66%) 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   #13272      +/-   ##
==========================================
- Coverage   91.57%   91.56%   -0.01%     
==========================================
  Files         522      522              
  Lines       29089    29162      +73     
==========================================
+ Hits        26639    26703      +64     
- Misses       1933     1939       +6     
- Partials      517      520       +3     

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