proxy: fix zero duration for non streaming responses (#723)

Updates #654

proxy/metrics_monitor.go

@@ -578,15 +578,11 @@ func newBodyCopier(w gin.ResponseWriter) *responseBodyCopier {
 		ResponseWriter: w,
 		body:           bodyBuffer,
 		tee:            io.MultiWriter(w, bodyBuffer),
+		start:          time.Now(),
 	}
 }
 
 func (w *responseBodyCopier) Write(b []byte) (int, error) {
-	if w.start.IsZero() {
-		w.start = time.Now()
-	}
-
-	// Single write operation that writes to both the response and buffer
 	return w.tee.Write(b)
 }
 

proxy/metrics_monitor_test.go

@@ -490,15 +490,11 @@ func TestMetricsMonitor_ResponseBodyCopier(t *testing.T) {
 		assert.Equal(t, string(testData), rec.Body.String())
 	})
 
-	t.Run("sets start time on first write", func(t *testing.T) {
+	t.Run("sets start time on creation", func(t *testing.T) {
 		rec := httptest.NewRecorder()
 		ginCtx, _ := gin.CreateTestContext(rec)
 		copier := newBodyCopier(ginCtx.Writer)
 
-		assert.True(t, copier.StartTime().IsZero())
-
-		copier.Write([]byte("test"))
-
 		assert.False(t, copier.StartTime().IsZero())
 	})