Metrics with Micrometer

@Service
@RequiredArgsConstructor
class OrderService {
    private final MeterRegistry registry;

    // Counter — increments on each call
    public Order place(CreateOrderRequest req) {
        Order order = orderRepo.save(new Order(req));
        registry.counter("orders.placed",
            "status",   "success",
            "region",   req.getRegion()
        ).increment();
        return order;
    }

    // Timer — records duration + count
    public PaymentResult charge(Order order) {
        return Timer.builder("payment.latency")
            .tag("provider", "stripe")
            .register(registry)
            .record(() -> stripeClient.charge(order));
    }
}

// Gauge — tracks a current value (register once, reads lambda on each scrape)
@Bean
MeterBinder queueDepthGauge(OrderQueue queue) {
    return registry -> Gauge.builder("orders.queue.depth", queue, OrderQueue::size)
            .description("Current orders waiting to be processed")
            .register(registry);
}

<!-- pom.xml -->
<dependency>
  <groupId>io.micrometer</groupId>
  <artifactId>micrometer-registry-prometheus</artifactId>
</dependency>

# application.properties
management.endpoints.web.exposure.include=prometheus,health,info
management.metrics.tags.application=order-service   # global tag on all metrics
management.metrics.tags.environment=production

# Auto-instrumented metrics (no code required):
# http.server.requests           — HTTP request latency + status
# hikaricp.connections.*         — connection pool stats
# jvm.memory.used                — heap/non-heap usage
# jvm.gc.pause                   — GC pause durations
# process.cpu.usage              — CPU utilisation
# kafka.consumer.fetch.latency.avg — Kafka consumer fetch time

# Prometheus scrape config
scrape_configs:
  - job_name: order-service
    metrics_path: /actuator/prometheus
    static_configs:
      - targets: ['order-service:8080']

// @Timed — auto-instruments method execution time
@RestController
class OrderController {

    @PostMapping("/orders")
    @Timed(value = "orders.create", extraTags = {"endpoint", "create-order"},
           description = "Time to create an order")
    ResponseEntity<Order> create(@RequestBody CreateOrderRequest req) {
        return ResponseEntity.status(201).body(orderService.place(req));
    }
}

// Grafana dashboard PromQL queries:
// Request rate (per second, 5m window)
rate(orders_placed_total[5m])

// P99 HTTP latency (Spring Boot auto-instruments http.server.requests)
histogram_quantile(0.99,
  rate(http_server_requests_seconds_bucket{uri="/orders",method="POST"}[5m]))

// Error rate
rate(http_server_requests_seconds_count{status=~"5.*"}[5m])
  / rate(http_server_requests_seconds_count[5m])

// Alert: error rate > 1%
- alert: HighErrorRate
  expr: |
    rate(http_server_requests_seconds_count{status=~"5.*"}[5m])
    / rate(http_server_requests_seconds_count[5m]) > 0.01
  for: 2m