func (g *Gauge) Add(delta float64) {
	g.gv.With(makeLabels(g.lvs...)).Add(delta)
}
Add is supported by Prometheus GaugeVecs.
package prometheus
import (
	"github.com/prometheus/client_golang/prometheus"
	"github.com/go-kit/kit/metrics"
	"github.com/go-kit/kit/metrics/internal/lv"
)
type Counter struct {
	cv  *prometheus.CounterVec
	lvs lv.LabelValues
}
func NewCounterFrom(opts prometheus.CounterOpts, labelNames []string) *Counter {
	cv := prometheus.NewCounterVec(opts, labelNames)
	prometheus.MustRegister(cv)
	return NewCounter(cv)
}
func NewCounter(cv *prometheus.CounterVec) *Counter {
	return &Counter{
		cv: cv,
	}
}
func (c *Counter) With(labelValues ...string) metrics.Counter {
	return &Counter{
		cv:  c.cv,
		lvs: c.lvs.With(labelValues...),
	}
}
func (c *Counter) Add(delta float64) {
	c.cv.With(makeLabels(c.lvs...)).Add(delta)
}
type Gauge struct {
	gv  *prometheus.GaugeVec
	lvs lv.LabelValues
}
func NewGaugeFrom(opts prometheus.GaugeOpts, labelNames []string) *Gauge {
	gv := prometheus.NewGaugeVec(opts, labelNames)
	prometheus.MustRegister(gv)
	return NewGauge(gv)
}
func NewGauge(gv *prometheus.GaugeVec) *Gauge {
	return &Gauge{
		gv: gv,
	}
}
func (g *Gauge) With(labelValues ...string) metrics.Gauge {
	return &Gauge{
		gv:  g.gv,
		lvs: g.lvs.With(labelValues...),
	}
}
func (g *Gauge) Set(value float64) {
	g.gv.With(makeLabels(g.lvs...)).Set(value)
}