Add offline Feature Flagging reference implementation

[leoromanovsky]

Motivation

Provide a small Java reference implementation for the startup-only offline Feature Flagging mode so the cross-SDK system-test contract can be exercised before the production implementation is reviewed. This draft is stacked on #11892 and intentionally keeps the offline API narrow: the application supplies complete UFC JSON bytes once at provider initialization.

Changes

• add Provider.Options.offlineConfiguration(byte[])
• route startup bytes through the bootstrap gateway to a dedicated offline configuration source
• parse UFC with the existing Java deserializer and publish the existing ServerConfiguration model
• reject missing, empty, malformed, or repeated offline configuration
• document Java startup usage and cover listener lifecycle, defensive copying, initialization order, malformed payload recovery, and repeated initialization

Decisions

• offline is a distinct startup-only source; it starts neither the agentless poller nor the FFE Remote Configuration subscription
• the public API owns the caller-facing bytes, while the agent-owned source performs parsing and dispatch
• the source registers once even if agent initialization is invoked repeatedly
• this is a provisional Java reference used to validate the system-test contract; all system-tests manifests remain disabled

flowchart LR
  A["#11892<br/>agentless + source selection"] --> B["This draft<br/>Java offline reference"]
  C["system-tests main<br/>disabled offline contract"] --> B
  B --> D["Provisional proof<br/>26 offline tests pass"]
  C --> E["Small per-SDK<br/>manifest enablement PRs"]

Loading

Validation

• ./gradlew :products:feature-flagging:feature-flagging-bootstrap:spotlessCheck :products:feature-flagging:feature-flagging-lib:spotlessCheck :products:feature-flagging:feature-flagging-api:spotlessCheck :products:feature-flagging:feature-flagging-agent:spotlessCheck
• ./gradlew :products:feature-flagging:feature-flagging-bootstrap:test :products:feature-flagging:feature-flagging-lib:test :products:feature-flagging:feature-flagging-api:test :products:feature-flagging:feature-flagging-agent:test — BUILD SUCCESSFUL
• ./gradlew :dd-java-agent:shadowJar :dd-trace-api:jar :products:feature-flagging:feature-flagging-api:jar — BUILD SUCCESSFUL
• locally built these three artifacts, installed them into system-tests, and force-ran the disabled offline contract: 26 passed in 37.82s

Companion system-tests draft: DataDog/system-tests#7305.

[datadog-official]

🎯 Code Coverage (details)
• Patch Coverage: 93.75%
• Overall Coverage: 57.21% (+0.00%)

_{This comment will be updated automatically if new data arrives.
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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	13.97 s	13.87 s	[+0.1%; +1.3%] (maybe worse)
startup:insecure-bank:tracing:Agent	12.99 s	13.01 s	[-0.8%; +0.5%] (no difference)
startup:petclinic:appsec:Agent	16.70 s	16.95 s	[-2.4%; -0.5%] (maybe better)
startup:petclinic:iast:Agent	17.01 s	17.00 s	[-0.8%; +0.9%] (no difference)
startup:petclinic:profiling:Agent	16.71 s	16.88 s	[-2.2%; +0.1%] (no difference)
startup:petclinic:sca:Agent	16.91 s	16.80 s	[-0.4%; +1.7%] (no difference)
startup:petclinic:tracing:Agent	16.14 s	16.13 s	[-1.0%; +1.1%] (no difference)

Commit: 8d654e23 · CI Pipeline · Benchmarking Platform UI

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