func GetOrCreateGauge(name string, f func() float64) *Gauge {
	return defaultSet.GetOrCreateGauge(name, f)
}
GetOrCreateGauge returns registered gauge with the given name
or creates new gauge if the registry doesn't contain gauge with
the given name.

name must be valid Prometheus-compatible metric with possible labels.
For instance,

* foo
* foo{bar="baz"}
* foo{bar="baz",aaa="b"}

The returned gauge is safe to use from concurrent goroutines.

Performance tip: prefer NewGauge instead of GetOrCreateGauge.
package metrics
import (
	"fmt"
	"io"
)
func NewGauge(name string, f func() float64) *Gauge {
	return defaultSet.NewGauge(name, f)
}
type Gauge struct {
	f func() float64
}
func (g *Gauge) Get() float64 {
	return g.f()
}
func (g *Gauge) marshalTo(prefix string, w io.Writer) {
	v := g.f()
	if float64(int64(v)) == v {
		fmt.Fprintf(w, "%s %d\n", prefix, int64(v))
	} else {
		fmt.Fprintf(w, "%s %g\n", prefix, v)
	}
}