What would you like to be added:
add tag for healthcheck_broken_total to identify which forward plugin failed

for example:

HealthcheckBrokenCount = promauto.NewCounter(prometheus.CounterOpts{
	Namespace: plugin.Namespace,
	Subsystem: "forward",
	Name:      "healthcheck_broken_total",
	Help:      "Counter of the number of complete failures of the healthchecks.",
}, []string{"to","zone"})

# HELP coredns_forward_healthcheck_broken_total Counter of the number of complete failures of the healthchecks.
# TYPE coredns_forward_healthcheck_broken_total counter
coredns_forward_healthcheck_broken_total{to="192.168.0.1:53,192.168.0.2:53",zone=".com"} 0

Why is this needed:
coredns_forward_healthcheck_broken_total added when forward plugin find all upstream is down. however there could be many forward plugin used for different block. we need some tag to identify which forward plugin failed.