Promeheus counter and Gauge example

StockManager.java

@Component
public class StockManager {
  @Autowired private MeterRegistry meterRegistry;
  private List<String> orders = new Vector<>();
  private Counter counter;
  private Gauge gauge;

  @PostConstruct
  public void init() {
    counter =
        Counter.builder("order_created")
            .description("number of orders created")
            .register(meterRegistry);
    gauge =
        Gauge.builder("stock.size", this, StockManager::getNumberOfItems)
            .description("Number of items in stocks")
            .register(meterRegistry);
  }

  public int getNumberOfItems() {
    return orders.size();
  }

  public void addItems(List<String> items) {
    orders.addAll(items);
    // measure gauge
    gauge.measure();
  }

  public List<String> getItem(int count) {
    List<String> items = new ArrayList<>(count);
    while (count > 0) {
      try {
        items.add(orders.remove(0));
      } catch (ArrayIndexOutOfBoundsException e) {
        break;
      }
      count -= 1;
    }
    // increase counter
    counter.increment();
    //  measure gauge
    gauge.measure();
    return items;
  }
}