Fix trace_sampled MDC flag for micrometer+OTel starter (PR 451)

[Copilot]

TraceSampledMdcHandler was placed in test sources (no-op in production) and used @ConditionalOnBean(Tracer.class) on a component-scanned @Configuration — which always fails because component-scan conditions are evaluated before autoconfiguration creates the Tracer bean.

Key fixes

TraceSampledMdcHandler — moved to main, fixed registration

• Moved from src/test → src/main so it actually runs in the application
• Registered as @Bean in LogConfiguration (not @Component) using ObjectProvider<Tracer> — resolves lazily at instantiation time, safe when tracing is disabled
• @Conditional(NoOpenTelemetryAgentCondition.class) — handler is skipped when the OTel Java agent is attached; the agent uses different MDC keys handled by StackdriverOpenTelemetryTraceMdcJsonProvider

@Bean
@Conditional(NoOpenTelemetryAgentCondition.class)
public TraceSampledMdcHandler traceSampledMdcHandler(ObjectProvider<Tracer> tracerProvider) {
    return new TraceSampledMdcHandler(tracerProvider.getIfAvailable()); // no-op if null
}

application.properties

• Added management.tracing.enabled=true and management.tracing.sampling.probability=1.0

WebLoggingFormatTest

• Asserts logging.googleapis.com/trace_sampled: true on every captured log statement, verifying the full chain: TraceSampledMdcHandler → MDC traceSampled → StackdriverMicrometerTraceMdcJsonProvider → JSON boolean field

LogStatement (test utility)

• Added getJsonPropertyBoolean(String name) for reading boolean JSON fields from captured log events

spring-boot-autoconfigure-gcp

• Removed leftover implementation 'io.micrometer:micrometer-tracing' — nothing in this module uses it