func PushAddCollectors(job, instance, url string, collectors ...Collector) error {
	return pushCollectors(job, instance, url, "POST", collectors...)
}
PushAddCollectors works like PushAdd, but it does not collect from the
default registry. Instead, it collects from the provided collectors. It is a
convenient way to push only a few metrics.
package prometheus
func Push(job, instance, url string) error {
	return defRegistry.Push(job, instance, url, "PUT")
}
func PushAdd(job, instance, url string) error {
	return defRegistry.Push(job, instance, url, "POST")
}
func PushCollectors(job, instance, url string, collectors ...Collector) error {
	return pushCollectors(job, instance, url, "PUT", collectors...)
}