Create BatchSpanProcessorBurstBenchmark.java

[adhamahmad]

Add JMH benchmark for BatchSpanProcessor burst absorption

Description

Part of #7508.

Follows the approach agreed in the issue: "Add a benchmark/test scenario that reproduces bursty high-throughput conditions and reports span drop behavior consistently" (agreed by @jack-berg).

Benchmark only: no production code or defaults change. It is a baseline for comparing opt-in alternatives later.

Changes

• Add BatchSpanProcessorBurstBenchmark. Each operation produces a burst (6,000 spans) and calls forceFlush(); to ensure the accepted spans are fully processed before the next burst.
• Simulate export latency (default 50 ms) with an exporter that completes asynchronously.
• Parameterize burst size, maxQueueSize, maxExportBatchSize and exporter latency.
• Report producedSpans, exportedSpans and droppedSpans as aux counters, updated during the measured operation. Drops come from the processor's internal telemetry (BatchSpanProcessorMetrics) and are checked against what the exporter completed; the benchmark fails if they disagree.

Testing

.\gradlew.bat --no-configuration-cache -PjmhIncludeSingleClass=BatchSpanProcessorBurstBenchmark :sdk:trace:jmh
.\gradlew.bat :sdk:trace:compileJmhJava :sdk:trace:checkstyleJmh

Per 6,000-span burst (aux counters are per-iteration totals, divided by bursts):

maxQueueSize	Exported	Dropped	Time per burst
2048 (default)	2,560	3,440 (57%)	~293 ms
8192	6,000	0	~672 ms

With the default queue, 57% of the burst is dropped. Increasing the queue to 8192 eliminates the drops for this workload, but requires draining all 6,000 spans rather than the 2,560 spans accepted with the default queue.

Environment: Windows, Temurin JDK 21.0.10. Absolute times are platform-dependent.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.12%. Comparing base (59581d4) to head (c20ade2).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8911      +/-   ##
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- Coverage     91.13%   91.12%   -0.02%     
+ Complexity    10703    10702       -1     
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  Files          1007     1007              
  Lines         28918    28918              
  Branches       3702     3702              
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- Hits          26355    26351       -4     
  Misses         1736     1736              
- Partials        827      831       +4     

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