Add ObservationAwareSpanThreadLocalAccessor constructor taking ObservationRegistry

[acktsap]

This pr adds constructor with ObservationRegistry parameter for api flexibility.

[marcingrzejszczak]

Hey, thanks for the PR. What's your use case that you need to add the ObservationRegistry on the OASTLA ?

[acktsap]

Hi @marcingrzejszczak

Just for decorating purpose.

e.g.

final ObservationRegistry observationRegistry = new ObservationRegistry() {

    private ObservationRegistry delegate = ObservationRegistry.create();

    @Override
    public Observation getCurrentObservation() {
        /* do something here */
        return delegate.getCurrentObservation();
    }

    @Override
    public Observation.Scope getCurrentObservationScope() {
        return delegate.getCurrentObservationScope();
    }

    @Override
    public void setCurrentObservationScope(Observation.Scope current) {
        delegate.setCurrentObservationScope(current);
    }

    @Override
    public ObservationConfig observationConfig() {
        return delegate.observationConfig();
    }
};

[marcingrzejszczak]

Technically i understand why, but why would you even want to decorate it?

[acktsap]

Not a critical one. I just want to track if it has currentObservation by log.

This is code snippet.

@Bean
fun observationRegistry(tracer: Tracer): ObservationRegistry {
    val delegate = ObservationRegistry.create().apply {
        observationConfig().observationHandler(DefaultTracingObservationHandler(tracer))
    }
    return object : ObservationRegistry by delegate {
        override fun getCurrentObservation(): Observation? {
            return delegate.currentObservation.also {
                logger.debug { "Current observation: $it" }
            }
        }
    }
}

@Bean
fun contextRegistry(observationRegistry: ObservationRegistry, tracer: Tracer): ContextRegistry {
    return ContextRegistry().apply {
        loadThreadLocalAccessors()
            .registerThreadLocalAccessor(ObservationAwareSpanThreadLocalAccessor(observationRegistry, tracer)) // use decorated one
    }
}

I want to track which value is return by this code.

Apart from that, it seems nice to taking ObservationRegistry parameter not just creating SimpleObservationRegistry.