What is Spring Boot's observability stack in Boot 4 with Micrometer and OpenTelemetry?

// Custom metrics with Micrometer:
@Service
public class OrderService {

    private final Counter orderCounter;
    private final Timer   orderTimer;

    public OrderService(MeterRegistry registry) {
        this.orderCounter = Counter.builder("orders.created")
            .description("Total orders created")
            .tag("environment", "prod")
            .register(registry);
        this.orderTimer = Timer.builder("orders.processing.time")
            .description("Time to process order")
            .register(registry);
    }

    public Order placeOrder(CreateOrderRequest request) {
        return orderTimer.record(() -> {
            Order order = doCreateOrder(request);
            orderCounter.increment();
            return order;
        });
    }
}

# application.yml: OpenTelemetry configuration (Boot 4)
management:
  opentelemetry:
    resource-attributes:
      service.name: order-service
      service.version: "2.1.0"
      deployment.environment: production
  tracing:
    sampling:
      probability: 0.1   # 10% sampling in production
  metrics:
    export:
      prometheus:
        enabled: true     # expose /actuator/prometheus

# Export traces to Jaeger / Zipkin / OTel Collector:
management.otlp.tracing.endpoint=http://otel-collector:4317/v1/traces