plugin/metrcs: fix datarace on listeners (#2835)

This fixes a data race on the listener(s) that get started in the
metrics plugins.

It also restore pkg/uniq to its former glory and removes and state being
carried in there; this means for metrics that registry.go was to
replicate that behavior *with* locking (as pkg/uniq doesn't do, or need
that).

Also renamed uniqAddr to just u, to make it slightly shorter.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/pkg/uniq/uniq.go

@@ -10,27 +10,22 @@ type U struct {
 type item struct {
 	state int          // either todo or done
 	f     func() error // function to be executed.
-	obj   interface{}  // any object to return when needed
 }
 
 // New returns a new initialized U.
 func New() U { return U{u: make(map[string]item)} }
 
-// Set sets function f in U under key. If the key already exists
-// it is not overwritten.
-func (u U) Set(key string, f func() error, o interface{}) interface{} {
-	if item, ok := u.u[key]; ok {
-		return item.obj
+// Set sets function f in U under key. If the key already exists it is not overwritten.
+func (u U) Set(key string, f func() error) {
+	if _, ok := u.u[key]; ok {
+		return
 	}
-	u.u[key] = item{todo, f, o}
-	return o
+	u.u[key] = item{todo, f}
 }
 
 // Unset removes the key.
 func (u U) Unset(key string) {
-	if _, ok := u.u[key]; ok {
-		delete(u.u, key)
-	}
+	delete(u.u, key)
 }
 
 // ForEach iterates for u executes f for each element that is 'todo' and sets it to 'done'.