Demonstrate SpanPrototype: bake constant span tags in decorators (phase 1b)

[dougqh]

What

Demonstrates SpanPrototype (#11894) by wiring BaseDecorator / ClientDecorator / ServerDecorator to bake a per-decorator prototype — the identity + constant tags each decorator contributes — the concrete usage behind the deck's slide examples.

Why

Shows the API in action: a decorator bakes its constant tags once (prototype() / prototypeBuilder()), the shape a span would adopt by copy. Also carries the decorator-coupled tests (the pure-API test stays in #11894).

Split of #11828

Part 2 of 2, stacked on #11894. Construction (buildSpan(SpanPrototype)) is #11834.

🤖 Generated with Claude Code

[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	13.96 s	13.95 s	[-0.7%; +0.9%] (no difference)
startup:insecure-bank:tracing:Agent	12.91 s	12.96 s	[-1.1%; +0.4%] (no difference)
startup:petclinic:appsec:Agent	16.92 s	16.95 s	[-1.1%; +0.7%] (no difference)
startup:petclinic:iast:Agent	17.08 s	17.07 s	[-0.7%; +0.9%] (no difference)
startup:petclinic:profiling:Agent	16.66 s	16.91 s	[-2.7%; -0.4%] (maybe better)
startup:petclinic:sca:Agent	17.01 s	16.91 s	[-0.4%; +1.5%] (no difference)
startup:petclinic:tracing:Agent	15.72 s	16.13 s	[-6.8%; +1.8%] (no difference)

Commit: 327afce3 · 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.

[dougqh]

Closing. This PR served its purpose as an end-to-end design validation of SpanPrototype (proving the constant-tag baking + construction path works from a decorator). It's a pure-additive demonstration (it adds prototype scaffolding alongside the existing afterStart stamping rather than replacing it), and its base carries an older revision of the SpanPrototype type/benchmark that #11894 has since reworked (folded construction path, uniform init* builder, integration-name carry, isEmptyValue).

Real usage will land as a fresh, tightly-scoped PR on top of #11894 — migrating a specific decorator's constant tags to a baked SpanPrototype (deleting the per-span setTag stamping) so the allocation win is exercised on a real path.

— Claude (via Claude Code)