Fix stale _dd.dsm.enabled tag after remote config toggles Data Streams

[dougqh]

What Does This Do

Rebuilds the tracer-wide span tags (mergedTracerTags) when remote config enables or disables Data Streams Monitoring, not only when the tracing tags change.

Motivation

ConfigSnapshot reuses the previous snapshot's mergedTracerTags whenever the tracing tags are unchanged. But withTracerTags also reads isDataStreamsEnabled() to set or remove _dd.dsm.enabled. So an RC update that changes only data_streams_enabled (TracingConfigPoller) left _dd.dsm.enabled stale on every new span:

• DSM enabled through RC: the tag stays missing;
• DSM disabled through RC: the tag stays set.

The tag only corrected itself once some later RC update also changed the tracing tags.

Fix: also compare isDataStreamsEnabled() in the reuse check. It only runs when a snapshot is built (on RC updates), never per span.

Additional Notes

• CoreTracerTest.remoteConfigDataStreamsToggleUpdatesTracerTags fails on master (expected: <1> but was: <null> after RC enables DSM) and passes with the fix.
• Full dd-trace-core locally: the only failures were two Docker-based tests (no Docker locally) and two timing tests (DDAgentWriterCombinedTest, PendingTraceBufferTest). Both timing tests pass when rerun on their own.
• Found by /perf-review on Keep DD_SPAN_TAGS when remote config overrides tracing tags (OTLP trace/resource-level split) #12937. It touches the same ConfigSnapshot block, so whichever lands second needs a small rebase.

Jira ticket: [PROJ-IDENT]

🤖 Generated with Claude Code

[datadog-prod-us1-3]

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 59.47% (-0.37%)

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[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	13.90 s	13.86 s	[-0.5%; +1.0%] (no difference)
startup:insecure-bank:tracing:Agent	12.81 s	12.92 s	[-1.6%; -0.1%] (maybe better)
startup:petclinic:appsec:Agent	17.06 s	16.92 s	[+0.0%; +1.7%] (maybe worse)
startup:petclinic:iast:Agent	16.83 s	16.96 s	[-1.6%; +0.1%] (no difference)
startup:petclinic:profiling:Agent	16.21 s	16.85 s	[-8.3%; +0.7%] (no difference)
startup:petclinic:sca:Agent	17.01 s	16.18 s	[+0.9%; +9.3%] (maybe worse)
startup:petclinic:tracing:Agent	16.17 s	16.08 s	[-0.3%; +1.4%] (no difference)

Commit: 3b8dd31a · CI Pipeline · Benchmarking Platform UI

---

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

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-19T21:13:17.182143Z	3b8dd31	Draft marked ready

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[datadog-prod-us1-3]

More details

The cache-reuse guard now accounts for DSM enablement, allowing remote-config updates to add or remove _dd.dsm.enabled even when tracing tags remain unchanged. The completed static review identified no reportable defects.

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