[#374] Add inner-ring event metrics

Signed-off-by: Alejandro Lopez <a.lopez@yadro.com>

pkg/innerring/metrics/metrics.go

@@ -0,0 +1,15 @@
+package metrics
+
+import "time"
+
+type Register interface {
+	SetEpoch(epoch uint64)
+	SetHealth(s int32)
+	AddEvent(d time.Duration, typ string, success bool)
+}
+
+type DefaultRegister struct{}
+
+func (DefaultRegister) SetEpoch(uint64)                      {}
+func (DefaultRegister) SetHealth(int32)                      {}
+func (DefaultRegister) AddEvent(time.Duration, string, bool) {}