fix(api): record client cancellations as 499 in metrics

Follow-up to 164f31a - applies the same client cancellation handling to
the metrics middleware. Cancelled requests are now recorded with
http.status_code=499 (nginx convention for client-closed-request) and
a client.canceled=true attribute, preventing them from inflating the
5xx error rate.

Author: dobrac

packages/api/internal/middleware/otel/metrics/middleware.go

@@ -1,6 +1,8 @@
 package metrics
 
 import (
+	"context"
+	"errors"
 	"fmt"
 	"strings"
 	"time"
@@ -71,7 +73,11 @@ func Middleware(meterProvider metric.MeterProvider, service string, options ...O
 				reqAttributes...,
 			)
 
-			if cfg.groupedStatus {
+			if errors.Is(ctx.Err(), context.Canceled) {
+				// 499 is the nginx convention for "client closed request before server responded"
+				resAttributes = append(resAttributes, semconv.HTTPStatusCodeKey.Int(499))
+				resAttributes = append(resAttributes, attribute.Bool("client.canceled", true))
+			} else if cfg.groupedStatus {
 				code := ginCtx.Writer.Status() / 100 * 100
 				resAttributes = append(resAttributes, semconv.HTTPStatusCodeKey.Int(code))
 			} else {