Observability: logs, metrics and tracing

@Service
class CheckoutService {
    private static final Logger log = LoggerFactory.getLogger(CheckoutService.class);

    OrderConfirmation checkout(String customerId, Cart cart) {
        log.atInfo()
           .setMessage("Checkout started")
           .addKeyValue("customerId", customerId)
           .addKeyValue("items", cart.items().size())
           .log();
        try {
            var confirmation = payments.charge(customerId, cart.totalCents());
            log.atInfo().setMessage("Order placed")
               .addKeyValue("orderId", confirmation.orderId()).log();
            return confirmation;
        } catch (PaymentDeclinedException e) {
            log.warn("Payment declined for customer {}", customerId, e);   // exception last: stack trace kept
            throw e;
        }
    }
}

// application.yml (Spring Boot 3.4+): JSON logs on the console
// logging.structured.format.console: ecs

@Service
class CheckoutService {
    private final Counter ordersPlaced;
    private final Timer paymentTimer;
    private final PaymentClient payments;

    CheckoutService(MeterRegistry registry, PaymentClient payments) {
        this.payments = payments;
        this.ordersPlaced = Counter.builder("shop.orders.placed")
                .description("Orders placed successfully")
                .register(registry);
        this.paymentTimer = Timer.builder("shop.payment.duration")
                .publishPercentileHistogram()                 // enables p95/p99 in Prometheus
                .register(registry);
    }

    OrderConfirmation checkout(String customerId, Cart cart) {
        var confirmation = paymentTimer.record(() -> payments.charge(customerId, cart.totalCents()));
        ordersPlaced.increment();
        return confirmation;
    }
}

// application.yml: expose /actuator/prometheus for scraping
// management.endpoints.web.exposure.include: health,prometheus

// build: io.micrometer:micrometer-tracing-bridge-otel + io.opentelemetry:opentelemetry-exporter-otlp
// application.yml:
//   management.tracing.sampling.probability: 0.1          # keep 10% of traces
//   management.otlp.tracing.endpoint: http://otel-collector:4318/v1/traces

@Service
class FraudCheck {
    private final ObservationRegistry observations;
    private final ScoringClient scoring;

    FraudCheck(ObservationRegistry observations, ScoringClient scoring) {
        this.observations = observations;
        this.scoring = scoring;
    }

    Score check(Order order) {
        return Observation.createNotStarted("fraud.check", observations)   // a span + a timer metric
                .lowCardinalityKeyValue("paymentMethod", order.paymentMethod().name())
                .observe(() -> scoring.score(order));
    }
}