plugin/metrics: add MustRegister function (#1648)

This registers the Collectors iff the metrics plugin has been loaded.
Safes a bunch of code in each and every plugin's setup code.

plugin/metrics/register.go

@@ -0,0 +1,23 @@
+package metrics
+
+import (
+	"github.com/coredns/coredns/core/dnsserver"
+
+	"github.com/mholt/caddy"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// MustRegister registers the prometheus Collectors when the metrics middleware is used.
+func MustRegister(c *caddy.Controller, cs ...prometheus.Collector) {
+	m := dnsserver.GetConfig(c).Handler("prometheus")
+	if m == nil {
+		return
+	}
+	x, ok := m.(*Metrics)
+	if !ok {
+		return
+	}
+	for _, c := range cs {
+		x.MustRegister(c)
+	}
+}