build.gradle

plugins {
    id 'java'
}

java {
    targetCompatibility JavaVersion.VERSION_17
    sourceCompatibility JavaVersion.VERSION_17
}

repositories.mavenCentral()
tasks.withType(Test).configureEach { useJUnitPlatform() } // also changing/ed soon? :(

dependencies {
    implementation platform('org.springframework.boot:spring-boot-dependencies:3.1.3')
    annotationProcessor platform('org.springframework.boot:spring-boot-dependencies:3.1.3')
    implementation platform('org.springframework.cloud:spring-cloud-dependencies:2022.0.4')

    annotationProcessor 'org.springframework.boot:spring-boot-configuration-processor'

    implementation 'org.springframework.boot:spring-boot-starter-web'
    implementation 'org.springframework.boot:spring-boot-starter-aop'
    implementation 'org.springframework.boot:spring-boot-starter-actuator'
    // implementation 'org.springframework.boot:spring-boot-starter-data-cassandra'
    // implementation 'org.springframework.boot:spring-boot-starter-data-cassandra-reactive'
    implementation 'org.springframework.boot:spring-boot-starter-validation'
    implementation 'org.springframework.cloud:spring-cloud-starter-circuitbreaker-reactor-resilience4j'
    // implementation 'org.springframework.kafka:spring-kafka'
    implementation 'io.micrometer:micrometer-tracing-bridge-brave' { exclude('group': 'io.zipkin.brave') }
    implementation 'io.micrometer:micrometer-tracing'

    runtimeOnly 'io.micrometer:micrometer-registry-prometheus'

    testImplementation 'org.springframework.boot:spring-boot-starter-test'
    testImplementation 'io.projectreactor:reactor-test'
    testImplementation 'org.springframework.boot:spring-boot-testcontainers'
    testImplementation 'org.testcontainers:cassandra'
    // testImplementation 'org.springframework.kafka:spring-kafka-test'
    // testImplementation 'org.testcontainers:junit-jupiter'
    // testImplementation 'org.testcontainers:kafka'
}

ReproducerApplication.java

package info.ankin.example;

import io.micrometer.tracing.handler.DefaultTracingObservationHandler;
import jakarta.annotation.PostConstruct;
import lombok.RequiredArgsConstructor;
import lombok.extern.slf4j.Slf4j;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.client.RestTemplateBuilder;
import org.springframework.context.annotation.Bean;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.client.RestTemplate;
import zipkin2.Call;
import zipkin2.codec.Encoding;
import zipkin2.reporter.Sender;

import java.util.List;

@SpringBootApplication
class ReproducerApplication {
    public static void main(String[] args) {
        // https://github.com/micrometer-metrics/tracing/wiki/Spring-Cloud-Sleuth-3.1-Migration-Guide#log-pattern
        System.setProperty("logging.pattern.level", "%5p [${spring.application.name:},%X{traceId:-},%X{spanId:-}]");
        SpringApplication.run(ReproducerApplication.class, args);
    }

    @Slf4j
    @RequiredArgsConstructor
    @RestController
    @RequestMapping("/")
    static class RaController {
        final RestTemplateBuilder restTemplateBuilder;
        final DefaultTracingObservationHandler a; // placed here to ensure MicrometerTracingAutoConfiguration was actually loaded
        RestTemplate restTemplate;

        @PostConstruct
        void init() {
            restTemplate = restTemplateBuilder.build();
        }

        @GetMapping("/")
        String abc() {
            /*
                this should print a trace
             */
            log.info("hello");
            /*
                what is running on localhost:3000?

                npx express-generator --no-view example-app-returns-headers
                cd example-app-returns-headers
                sed -i -e '/static/s/^/\/\//' -e "/app.use('\//s/^/\/\//" app.js
                echo >> app.js
                echo "app.use('/', function returnHeaders(req, res, next) { res.send(req.headers); });" >> app.js
                npm i
                npm run start
             */
            return restTemplate.getForObject("http://localhost:3000/", String.class);
        }
    }

    @Bean
    Sender noopSender() {
        return new Sender() {
            @Override
            public Encoding encoding() {
                return Encoding.JSON;
            }

            @Override
            public int messageMaxBytes() {
                return Integer.MAX_VALUE;
            }

            @Override
            public int messageSizeInBytes(List<byte[]> encodedSpans) {
                return 0;
            }

            @Override
            public Call<Void> sendSpans(List<byte[]> encodedSpans) {
                return Call.create(null);
            }
        };
    }
}