plugin/metrics: Added New func (#1309)

If external plugins wanted to extend metrics there was no way since
zoneNames couldn't be initialized. Now plugins can call New to get an
instance of Metrics that they can extend.

plugin/metrics/metrics.go

@@ -36,6 +36,11 @@ type Metrics struct {
 	zoneMu    sync.RWMutex
 }
 
+// New returns a new instance of Metrics with the given address
+func New(addr string) *Metrics {
+	return &Metrics{Addr: addr, zoneMap: make(map[string]bool)}
+}
+
 // AddZone adds zone z to m.
 func (m *Metrics) AddZone(z string) {
 	m.zoneMu.Lock()

plugin/metrics/metrics_test.go

@@ -13,7 +13,7 @@ import (
 )
 
 func TestMetrics(t *testing.T) {
-	met := &Metrics{Addr: "localhost:0", zoneMap: make(map[string]bool)}
+	met := New("localhost:0")
 	if err := met.OnStartup(); err != nil {
 		t.Fatalf("Failed to start metrics handler: %s", err)
 	}

plugin/metrics/setup.go

@@ -42,10 +42,7 @@ func setup(c *caddy.Controller) error {
 }
 
 func prometheusParse(c *caddy.Controller) (*Metrics, error) {
-	var (
-		met = &Metrics{Addr: addr, zoneMap: make(map[string]bool)}
-		err error
-	)
+	var met = New(defaultAddr)
 
 	defer func() {
 		uniqAddr.SetAddress(met.Addr)
@@ -73,7 +70,7 @@ func prometheusParse(c *caddy.Controller) (*Metrics, error) {
 			return met, c.ArgErr()
 		}
 	}
-	return met, err
+	return met, nil
 }
 
 var uniqAddr addrs
@@ -91,8 +88,8 @@ func (a *addrs) SetAddress(addr string) {
 	a.a[addr] = todo
 }
 
-// Addr is the address the where the metrics are exported by default.
-const addr = "localhost:9153"
+// defaultAddr is the address the where the metrics are exported by default.
+const defaultAddr = "localhost:9153"
 
 const (
 	todo = 1