Feature request: Add sampling flag to MDC

[jamesmoessis]

The Slf4jEventListener in this repo is what adds the trace ID and the span ID to the logs.

It's useful to add the sampling flag to the logs. An example use case being a developer looking at their logs and being able to filter the logs which would have a trace present in their tracing system.

This could be added to Slf4jEventListener as an optional field.

Would love the maintainer's thoughts. Would be happy to implement myself.

I'm coming from the perspective of the otel bridge but of course I assume the same thing could be implemented for brave.

[bams22]

It looks like a good change. I'm sure many people will appreciate it. Why not add this change made by @jamesmoessis without waiting for the comprehensive solution that @jamesmoessis mentioned in his PR?

[jamesmoessis]

@bams22, Spoke to the maintainer @jonatan-ivanov didn't want the change to go in because they are working on an adjacent feature set that solves this for more than just sampled flag

[jamesmoessis]

@bams22 If you want to add this into your own project, register a bean that does it. This will add onto existing event listeners.

I can share with you my one:

/**
 * Reacts to tracing context changes.
 * Annotates a boolean on the MDC to indicate whether the trace context is sampled.
 * Essentially a {@link io.micrometer.tracing.otel.bridge.Slf4JEventListener} but
 * for sampling flag instead of traceID and spanID.
 */
@Component
final class MdcSampledEventListener implements EventListener {
    private final String sampledKey;

    /**
     * @param sampledKey the key to be put on the MDC
     */
    MdcSampledEventListener(String sampledKey) {
        this.sampledKey = sampledKey;
    }

    private void onScopeAttached(EventPublishingContextWrapper.ScopeAttachedEvent event) {
        Span span = event.getSpan();
        if (span != null) {
            MDC.put(this.sampledKey, Boolean.toString(span.getSpanContext().isSampled()));
        }
    }

    private void onScopeRestored(EventPublishingContextWrapper.ScopeRestoredEvent event) {
        Span span = event.getSpan();
        if (span != null) {
            MDC.put(this.sampledKey, Boolean.toString(span.getSpanContext().isSampled()));
        }

    }

    private void onScopeClosed(EventPublishingContextWrapper.ScopeClosedEvent event) {
        MDC.remove(this.sampledKey);
    }

    public void onEvent(Object event) {
        if (event instanceof EventPublishingContextWrapper.ScopeAttachedEvent) {
            this.onScopeAttached((EventPublishingContextWrapper.ScopeAttachedEvent) event);
        } else if (event instanceof EventPublishingContextWrapper.ScopeClosedEvent) {
            this.onScopeClosed((EventPublishingContextWrapper.ScopeClosedEvent) event);
        } else if (event instanceof EventPublishingContextWrapper.ScopeRestoredEvent) {
            this.onScopeRestored((EventPublishingContextWrapper.ScopeRestoredEvent) event);
        }
    }
}