feat(server): wire enriched logger into composition root and request context (S03 Step 3)

What

Step 3 (final) of the S03 logging foundation: wire the enriched logger into the running service. This is the feat step where the service actually honors log.level/log.format, samples access logs, and enriches every record with context fields — Steps 1 (config) and 2 (handler chain) are inert until this lands.

• Composition root (cmd/artifact-registry/main.go): map cfg.Log into app.Config.Log (via appLogConfig) so app.App builds the base handler at the configured level/format, then build the enriched logger by wrapping a.Logger().Handler() with the Step 2 chain (logging.NewLogger(...), sampler keyed by samplerConfig(cfg.Log.AccessLog)) and hand it to server.New. Wrapping (not building a fresh handler) preserves app.App's baked-in version/commit/build-date attributes.
• Request-context logger (internal/server): ContextLoggerMiddleware stores the enriched logger via log.WithLogger, installed as the outermost application middleware in buildMiddleware (inside LabKit's correlation middleware, outside the mux), so log.FromContext(ctx) returns it at every handler with correlation_id already in context.
• Docs (docs/dev/logging.md): corrected the LabKit-usage guidance (log.AppendFields not log.WithFields; status not status_code; error_message not error; uri not path; the no-auto-stamp note) — paired with the implementation that introduces these primitives.

Plan: docs/plans/2026-06-02-s03-logging.md, Step 3. Spec: docs/specs/S03-observability.md.

Spec coverage

This step owns AC #1 and the applied-before-serving half of AC #7 (closed); the remaining ACs are owned by Steps 1/2 and listed for completeness.

Acceptance criteria

#	Criterion	Tests
AC-1	Base handler writes JSON to stderr by default; level honored; version/commit/build-date survive wrap	TestWiring_WrapAppLogger, TestWiring_ConfigLogDrivesAppLevel
AC-2	context-field handler attaches correlation_id/trace_id/span_id; no-op when absent	Step 2: TestHandler_CorrelationPresent, TestHandler_TraceSpanPresent, TestHandler_NoneWhenAbsent
AC-3	sampler always logs status>=400 and slow responses	Step 2: TestSampler_ErrorAlwaysLogged, TestSampler_SlowAlwaysLogged
AC-4	success_fraction=0.0 drops fast 2xx/3xx, keeps errors/slow	Step 2: TestSampler_FastSuccessDropped, TestSampler_ErrorAlwaysLogged
AC-5	access_log.disabled=true emits nothing	Step 2: TestSampler_Disabled
AC-6	wide-event emitter carries outcome+duration_s; context fields by chain	Step 2: TestWideEvent_BaselineFields
AC-7	LogConfig loaded/validated and applied before serving; FromContext returns enriched logger	Step 1: internal/config TestLog_*. Applied-before-serving (this step): TestWiring_ContextLogger, TestContextLoggerMiddleware_InstallsLogger, TestContextLoggerMiddleware_EnrichesFromContext
AC-8	success_fraction bounds + explicit-0.0 vs unset; level/format in: rejection	Step 1: internal/config TestLog_FractionOutOfRange, TestLog_ExplicitZeroFraction, TestLog_InvalidLevel

Sampler-through-the-assembled-logger (plan Step 3 Tests)

Behavior	Tests
Logger handed to the server is the sampling logger (error preserved)	TestWiring_AccessLogSampling
Wide events are never dropped by the access sampler	TestWiring_NonAccessWideEventNotSampled

Plus direct mapping tests for the composition-root helpers: TestAppLogConfig (level + UseTextFormat) and TestSamplerConfig (Disabled / SuccessFraction / SlowThreshold).

Error cases

#	Condition	Tests
E-1	Log pipeline saturated → success_fraction<1.0 caps volume	Indirect: TestWiring_AccessLogSampling drop path; the service does not observe downstream pressure
E-2	access_log.disabled=true → no records, rules bypassed	Step 2: TestSampler_Disabled
E-3	log.level/log.format omitted or "" → default applied	Step 1: TestLog_OmittedBlock, TestLog_EmptyEnums
E-4	log.level/log.format outside in: set → protovalidate rejection	Step 1: TestLog_InvalidLevel
E-5	invalid slow_threshold → startup error naming the field	Step 1: TestLog_InvalidSlowThreshold

Security considerations

#	Concern	Tests
S-1	No secrets in logs (per-event schema)	Enforced by the typed-emitter convention; no concrete emitter in S03 (owned by format specs). Not exercised here.
S-2	X-Request-ID echo / correlation sanitization	LabKit-owned (CorrelationIDMiddleware regex). Not tested here.

Review notes

This branch went through /validate-step → /review-branch → /validate-step; all findings are resolved:

• Spec amendment included (out of plan Files, deliberate). /review-branch found that the spec's LabKit-mapping table claimed log.format overrides GITLAB_LOG_FORMAT unconditionally, but LabKit applies the UseTextFormat override only when true — so log.format: json does not override an env-set GITLAB_LOG_FORMAT=text. The implementation is correct (it's a LabKit library behavior), so docs/specs/S03-observability.md:377 was corrected to document the one-directional precedence rather than changing code. Flagging that this spec edit rides this MR rather than a standalone spec MR.
• Data race fixed. The build-info wiring tests pinned LabKit's process-global app.Version/Commit/Date per-test under t.Parallel(), racing reads in sibling tests. Now pinned once in init(); verified clean under go test -race -count=3.
• Added direct appLogConfig/samplerConfig tests and corrected a misleading helper comment.

Diff size

Test- and docs-dominated: ~64 LOC of reviewable production code (main.go + middleware.go + server.go); the rest is wiring tests and the paired docs/spec corrections.