document how to use micrometer tracing with spring starter

[zeitlinger]

It should work by adding runtimeOnly "io.micrometer:micrometer-tracing-bridge-otel" to the dependencies.

Not sure if this should be included in the starter by default.

[zeitlinger]

Here's how to do it:

Dependencies:

implementation "io.micrometer:micrometer-tracing-bridge-otel"

Enable support for @NewSpan - which is from micrometer tracing.

management:
  observations:
    annotations:
      enabled: true

Use @NewSpan

@Service
public class DemoService {
    @NewSpan
    public String sayHello() {
        return "hello LGTM";
    }
}

I didn't test any more advanced features of micrometer tracing - so it's possible there are some rough edges.

Config to suppress double spans:

	@Bean
	public DefaultTracingObservationHandler defaultTracingObservationHandler(Tracer tracer) {
		return new DefaultTracingObservationHandler(tracer) {
			@Override
			public void onStop(Observation.Context context) {
				// no-op
			}

			@Override
			public void onStart(Observation.Context context) {
				// no-op
			}
		};
	}

	@Bean
	public PropagatingReceiverTracingObservationHandler<?> propagatingReceiverTracingObservationHandler(Tracer tracer,
																										Propagator propagator) {
		return new PropagatingReceiverTracingObservationHandler<>(tracer, propagator) {
            @Override
            public void onError(ReceiverContext context) {
                // no-op
            }

            @Override
            public void onStop(ReceiverContext context) {
                // no-op
            }
        };
	}

	@Bean
	public PropagatingSenderTracingObservationHandler<?> propagatingSenderTracingObservationHandler(Tracer tracer,
																									Propagator propagator) {
		return new PropagatingSenderTracingObservationHandler<>(tracer, propagator) {
			@Override
			public void onError(SenderContext context) {
				// no-op
			}

			@Override
			public void onStop(SenderContext context) {
				// no-op
			}
		};
	}

before (with double spans):

after:

[zeitlinger]

@jeanbisutti should we document this? should we include the "suppress double spans" code in the starter?

[trask]

I don't believe supporting micrometer-tracing-bridge-otel is in scope for us, so would prefer not to document it

in the past the spring folks have suggested they may donate the bridge to opentelemetry-java-contrib repository, in which case we could document it there

[jeanbisutti]

@zeitlinger @trask We could discuss during the Java SIG and potentially close this issue