[chore][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%

I am surprised to see BatchMetricProcessor-16 performance regress here - does anyone have an idea of why that might be? I'll dig into it a bit more as well.

Correctness

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

[codecov]

Codecov Report

❌ Patch coverage is 97.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.96%. Comparing base (8b73477) to head (4173d05).
⚠️ Report is 146 commits behind head on main.

Files with missing lines	Patch %	Lines
processor/batchprocessor/splitmetrics.go	95.74%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #13272   +/-   ##
=======================================
  Coverage   91.95%   91.96%           
=======================================
  Files         529      529           
  Lines       31446    31513   +67     
=======================================
+ Hits        28917    28981   +64     
- Misses       1998     2000    +2     
- Partials      531      532    +1     

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[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[jmacd]

There has been a long effort to replace this code with new code.
#13547
It may have the same defect, see #8122.

[jmacd]

open-telemetry/opentelemetry.io#7244

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[github-actions]

Closed as inactive. Feel free to reopen if this PR is still being worked on.