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/proxy/metrics.go

@@ -25,15 +25,6 @@ var (
 	}, []string{"proto", "proxy_proto", "family", "to"})
 )
 
-// OnStartupMetrics sets up the metrics on startup. This is done for all proxy protocols.
-func OnStartupMetrics() error {
-	metricsOnce.Do(func() {
-		prometheus.MustRegister(RequestCount)
-		prometheus.MustRegister(RequestDuration)
-	})
-	return nil
-}
-
 // familyToString returns the string form of either 1, or 2. Returns
 // empty string is not a known family
 func familyToString(f int) string {
@@ -46,4 +37,4 @@ func familyToString(f int) string {
 	return ""
 }
 
-var metricsOnce sync.Once
+var once sync.Once

plugin/proxy/setup.go

@@ -3,6 +3,7 @@ package proxy
 import (
 	"github.com/coredns/coredns/core/dnsserver"
 	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/metrics"
 
 	"github.com/mholt/caddy"
 )
@@ -28,7 +29,19 @@ func setup(c *caddy.Controller) error {
 		return P
 	})
 
-	c.OnStartup(OnStartupMetrics)
+	c.OnStartup(func() error {
+		once.Do(func() {
+			m := dnsserver.GetConfig(c).Handler("prometheus")
+			if m == nil {
+				return
+			}
+			if x, ok := m.(*metrics.Metrics); ok {
+				x.MustRegister(RequestCount)
+				x.MustRegister(RequestDuration)
+			}
+		})
+		return nil
+	})
 
 	for i := range upstreams {
 		u := upstreams[i]