Metrics registered on wrong prometheus registry (#2246)

* - UT on metrics verifying that all plugins of all blocs have their metrics collectors declared

* - fix error msg

* - redirect Registry of metric to the one that handle the listener
- allow duplicate of metrics collector on the same Registry (case of same plugin in 2 blocs listening metrics on the same address)

* - fix change of signature

* - ensure cleaning metrics before starting the test (metrics collectors are global vars .. and re-used by several tests)

* - I think I fixed this test. Ensure correct mn of hits and clean metrics before test.

* - fix typo in error msg - proposed at review

* - fix typo in comment

* - remove ResetMetrics functions
- change a way to test the numeric metrics : get the diff between begin and end of test

* - oops. removing debug logs

plugin/metrics/setup.go

@@ -31,6 +31,13 @@ func setup(c *caddy.Controller) error {
 		return plugin.Error("prometheus", err)
 	}
 
+	// register the metrics to its address (ensure only one active metrics per address)
+	obj := uniqAddr.Set(m.Addr, m.OnStartup, m)
+	//propagate the real active Registry to current metrics
+	if om, ok := obj.(*Metrics); ok {
+		m.Reg = om.Reg
+	}
+
 	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
 		m.Next = next
 		return m
@@ -55,10 +62,6 @@ func setup(c *caddy.Controller) error {
 func prometheusParse(c *caddy.Controller) (*Metrics, error) {
 	var met = New(defaultAddr)
 
-	defer func() {
-		uniqAddr.Set(met.Addr, met.OnStartup)
-	}()
-
 	i := 0
 	for c.Next() {
 		if i > 0 {