Fix perfsprint lint: use errors.New for static message

Author: ev-shindin

internal/metrics/metrics.go

@@ -2,6 +2,7 @@ package metrics
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"os"
 
@@ -239,7 +240,7 @@ func (m *MetricsEmitter) EmitSaturationMetrics(
 ) error {
 	if saturationUtilization == nil || spareCapacity == nil || requiredCapacity == nil ||
 		kvCacheTokensUsed == nil || kvCacheTokensTotal == nil {
-		return fmt.Errorf("saturation metrics not initialized")
+		return errors.New("saturation metrics not initialized")
 	}
 
 	accelLabels := prometheus.Labels{