metrics: correctly register all metrics (#1335)

After initial startup, see if prometheus is loaded and if so, register
our metrics with it.
Stop doing the init() func and just use the sync.Once so we don't double
registrer our metrics.

plugin/dnssec/handler.go

@@ -1,6 +1,8 @@
 package dnssec
 
 import (
+	"sync"
+
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/request"
 
@@ -42,28 +44,28 @@ func (d Dnssec) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 var (
 	cacheSize = prometheus.NewGaugeVec(prometheus.GaugeOpts{
 		Namespace: plugin.Namespace,
-		Subsystem: subsystem,
+		Subsystem: "dnssec",
 		Name:      "cache_size",
 		Help:      "The number of elements in the dnssec cache.",
 	}, []string{"type"})
 
 	cacheCapacity = prometheus.NewGaugeVec(prometheus.GaugeOpts{
 		Namespace: plugin.Namespace,
-		Subsystem: subsystem,
+		Subsystem: "dnssec",
 		Name:      "cache_capacity",
 		Help:      "The dnssec cache's capacity.",
 	}, []string{"type"})
 
 	cacheHits = prometheus.NewCounter(prometheus.CounterOpts{
 		Namespace: plugin.Namespace,
-		Subsystem: subsystem,
+		Subsystem: "dnssec",
 		Name:      "cache_hits_total",
 		Help:      "The count of cache hits.",
 	})
 
 	cacheMisses = prometheus.NewCounter(prometheus.CounterOpts{
 		Namespace: plugin.Namespace,
-		Subsystem: subsystem,
+		Subsystem: "dnssec",
 		Name:      "cache_misses_total",
 		Help:      "The count of cache misses.",
 	})
@@ -72,11 +74,4 @@ var (
 // Name implements the Handler interface.
 func (d Dnssec) Name() string { return "dnssec" }
 
-const subsystem = "dnssec"
-
-func init() {
-	prometheus.MustRegister(cacheSize)
-	prometheus.MustRegister(cacheCapacity)
-	prometheus.MustRegister(cacheHits)
-	prometheus.MustRegister(cacheMisses)
-}
+var once sync.Once