chore: simplify prometheus check (#3672)

core/metric/histogram.go

@@ -3,7 +3,6 @@ package metric
 import (
 	prom "github.com/prometheus/client_golang/prometheus"
 	"github.com/zeromicro/go-zero/core/proc"
-	"github.com/zeromicro/go-zero/core/prometheus"
 )
 
 type (
@@ -54,11 +53,9 @@ func NewHistogramVec(cfg *HistogramVecOpts) HistogramVec {
 }
 
 func (hv *promHistogramVec) Observe(v int64, labels ...string) {
-	if !prometheus.Enabled() {
-		return
-	}
-
-	hv.histogram.WithLabelValues(labels...).Observe(float64(v))
+	update(func() {
+		hv.histogram.WithLabelValues(labels...).Observe(float64(v))
+	})
 }
 
 func (hv *promHistogramVec) close() bool {