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/cache/handler.go

@@ -1,6 +1,7 @@
 package cache
 
 import (
+	"sync"
 	"time"
 
 	"github.com/coredns/coredns/plugin"
@@ -84,38 +85,31 @@ func (c *Cache) get(now time.Time, qname string, qtype uint16, do bool) (*item,
 var (
 	cacheSize = prometheus.NewGaugeVec(prometheus.GaugeOpts{
 		Namespace: plugin.Namespace,
-		Subsystem: subsystem,
+		Subsystem: "cache",
 		Name:      "size",
 		Help:      "The number of elements in the cache.",
 	}, []string{"type"})
 
 	cacheCapacity = prometheus.NewGaugeVec(prometheus.GaugeOpts{
 		Namespace: plugin.Namespace,
-		Subsystem: subsystem,
+		Subsystem: "cache",
 		Name:      "capacity",
 		Help:      "The cache's capacity.",
 	}, []string{"type"})
 
 	cacheHits = prometheus.NewCounterVec(prometheus.CounterOpts{
 		Namespace: plugin.Namespace,
-		Subsystem: subsystem,
+		Subsystem: "cache",
 		Name:      "hits_total",
 		Help:      "The count of cache hits.",
 	}, []string{"type"})
 
 	cacheMisses = prometheus.NewCounter(prometheus.CounterOpts{
 		Namespace: plugin.Namespace,
-		Subsystem: subsystem,
+		Subsystem: "cache",
 		Name:      "misses_total",
 		Help:      "The count of cache misses.",
 	})
 )
 
-const subsystem = "cache"
-
-func init() {
-	prometheus.MustRegister(cacheSize)
-	prometheus.MustRegister(cacheCapacity)
-	prometheus.MustRegister(cacheHits)
-	prometheus.MustRegister(cacheMisses)
-}
+var once sync.Once