Id-keyed TagMap.set(long) + insertion-comparison benchmark

[dougqh]

What

On top of the bloom fast-path (#11900):

1. TagMap.set(long id, Object) — the id-keyed insertion path: given a resolved KnownTags.*_ID, store densely and skip keyOf name resolution. Sole TagMap impl (OptimizedTagMap); the id must be a stored known id (custom names keep the name setters).
2. TagMapInsertionComparisonBenchmark — HashMap / 1.0-bucket / 2.0-name / 2.0-id, alloc + throughput; the numbers behind the deck's slides 3/7/8.

Results (idle box, -prof gc -f 5 -wi 5 -i 5; persisted in the benchmark header)

arm	alloc B/op (7 / 12)	thrpt vs HashMap (7 / 12)
hashMap	352 / 512	1.00× / 1.00×
tagMapById	184 / 408	0.99× / 0.63×
tagMapByName	184 / 408	0.66× / 0.50×
tagMapCustom (1.0 bucket)	416 / 712	0.59× / 0.46×

• Alloc: dense ~half of HashMap; id == name (the bloom/dense win is CPU, not alloc).
• Throughput: id-insertion reaches HashMap parity at typical tag counts (0.99× @7) and beats the 1.0 bucket path 1.4–1.7×. The 12-tag gap is a sizing/resize artifact (SpanPrototype's right-sizing / a larger init cap closes it), not a scan — the tags don't collide under the bloom's fieldPos & 63.

Scope

Minimal id API (TagMap level). Full AgentSpan.setTag(long) + the KnownTags call-site migration are follow-ons. set(long) has no production caller yet — by design, it lands with its benchmark evidence ahead of the migration that consumes it (each PR stands alone in sequence).

Stacking

#11814 (dense storage) → #11900 (bloom) → this (id API + comparison benchmark).

Follow-ups

TagMap.Ledger + SpanBuilder id support; per-type graph coloring; master run to pin true-1.0 (no keyOf).

🤖 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	13.98 s	[-0.9%; +0.6%] (no difference)
startup:insecure-bank:tracing:Agent	12.92 s	13.03 s	[-1.7%; +0.1%] (no difference)
startup:petclinic:appsec:Agent	16.95 s	16.87 s	[-0.6%; +1.6%] (no difference)
startup:petclinic:iast:Agent	16.90 s	16.94 s	[-1.1%; +0.7%] (no difference)
startup:petclinic:profiling:Agent	16.06 s	16.83 s	[-9.1%; -0.1%] (maybe better)
startup:petclinic:sca:Agent	16.97 s	16.78 s	[+0.1%; +2.3%] (maybe worse)
startup:petclinic:tracing:Agent	16.15 s	16.07 s	[-0.5%; +1.4%] (no difference)

Commit: 7e3ed31f · CI Pipeline · Benchmarking Platform UI

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[datadog-prod-us1-6]

🎯 Code Coverage (details)
• Patch Coverage: 91.46%
• Overall Coverage: 57.31% (-0.01%)

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