Metrics listener fix (#2036)

* Create test to verify correct listener behavior

* Create Unset function to remove todo items

* Reset address for prometheus listener before restarting

* Add inline documentation for Unset function

* Make shutdownTimeout a constant and change to five seconds

* Revert ForEach behavior in uniq package

plugin/pkg/uniq/uniq.go

@@ -24,6 +24,13 @@ func (u U) Set(key string, f func() error) {
 	u.u[key] = item{todo, f}
 }
 
+// Unset removes the 'todo' associated with a key
+func (u U) Unset(key string) {
+	if _, ok := u.u[key]; ok {
+		delete(u.u, key)
+	}
+}
+
 // SetTodo sets key to 'todo' again.
 func (u U) SetTodo(key string) {
 	v, ok := u.u[key]