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_test.go

@@ -4,7 +4,7 @@ import "testing"
 
 func TestForEach(t *testing.T) {
 	u, i := New(), 0
-	u.Set("test", func() error { i++; return nil }, nil)
+	u.Set("test", func() error { i++; return nil })
 
 	u.ForEach()
 	if i != 1 {