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()