Reduce instrumentation test shard duration

[cgcote]

Summary

• increase the blocking AMD64 instrumentation test matrix from 8 to 12 shards
• assign instrumentation projects with a deterministic longest-processing-time allocator
• seed the allocator with CI Visibility durations for the known heavy projects and a conservative fallback for the rest
• scope duration weighting to this 12-way job; ARM64 and other matrices retain normal hashing

Motivation

CI Visibility showed that 12-way hash sharding reduced the slowest instrumentation shard from 37m49s to only 35m07s, with a 2.9x spread between the slowest and fastest shards. Moving JDBC alone shortened its old shard but merely moved the critical path to another shard.

This experiment separates the four dominant workloads (Armeria, JDBC, Spring WebFlux, and Lettuce) and balances the remaining measured projects by expected duration.

Validation

• ./gradlew :buildSrc:spotlessCheck
• all 12 instrumentation shards select tasks successfully in Gradle dry runs
• Armeria, JDBC, Spring WebFlux, and Lettuce resolve to distinct shards
• git diff --check

Notes

This PR contains one CI-only commit based on master.

[dd-octo-sts]

🟢 Java Benchmark SLOs — All performance SLOs passed

Suite	Status
Startup	🟢 pass

SLO thresholds are defined here based on automatically generated metrics. A warning is raised when results are within 5% of the threshold.

PR vs. master results

Scenario	Candidate	master	Δ (95% CI of mean)
startup:insecure-bank:iast:Agent	14.86 s	14.79 s	[-0.4%; +1.3%] (no difference)
startup:insecure-bank:tracing:Agent	13.75 s	13.91 s	[-1.9%; -0.4%] (maybe better)
startup:petclinic:appsec:Agent	17.25 s	17.16 s	[-0.4%; +1.5%] (no difference)
startup:petclinic:iast:Agent	16.93 s	17.05 s	[-1.5%; +0.1%] (no difference)
startup:petclinic:profiling:Agent	16.60 s	16.75 s	[-2.1%; +0.4%] (no difference)
startup:petclinic:sca:Agent	17.25 s	16.99 s	[+0.5%; +2.4%] (maybe worse)
startup:petclinic:tracing:Agent	16.11 s	16.19 s	[-1.4%; +0.4%] (no difference)

Commit: cdc2b15c · CI Pipeline · Benchmarking Platform UI

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Load and DaCapo benchmarks can be triggered manually in the GitLab pipeline. Results will appear in the Benchmarking Platform UI after completion.