feat: mysql and redis metric support (#2355)

* feat: mysql and redis metric support

* feat: mysql and redis metric support

* feat: mysql and redis metric support

Co-authored-by: dawn.zhou <dawn.zhou@yijinin.com>

core/metric/histogram.go

@@ -1,8 +1,10 @@
 package metric
 
 import (
-	prom "github.com/prometheus/client_golang/prometheus"
 	"github.com/zeromicro/go-zero/core/proc"
+	"github.com/zeromicro/go-zero/core/prometheus"
+
+	prom "github.com/prometheus/client_golang/prometheus"
 )
 
 type (
@@ -53,6 +55,10 @@ func NewHistogramVec(cfg *HistogramVecOpts) HistogramVec {
 }
 
 func (hv *promHistogramVec) Observe(v int64, labels ...string) {
+	if !prometheus.Enabled() {
+		return
+	}
+
 	hv.histogram.WithLabelValues(labels...).Observe(float64(v))
 }