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/metrics/registry.go

@@ -0,0 +1,28 @@
+package metrics
+
+import (
+	"sync"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+type reg struct {
+	sync.RWMutex
+	r map[string]*prometheus.Registry
+}
+
+func newReg() *reg { return &reg{r: make(map[string]*prometheus.Registry)} }
+
+// update sets the registry if not already there and returns the input. Or it returns
+// a previous set value.
+func (r *reg) getOrSet(addr string, pr *prometheus.Registry) *prometheus.Registry {
+	r.Lock()
+	defer r.Unlock()
+
+	if v, ok := r.r[addr]; ok {
+		return v
+	}
+
+	r.r[addr] = pr
+	return pr
+}