Metrics reload (#1586)

* wip

* plugin/metrics: fix reload behavior

Fixes #1472

plugin/metrics/README.md

@@ -33,6 +33,8 @@ Extra labels used are:
 If monitoring is enabled, queries that do not enter the plugin chain are exported under the fake
 name "dropped" (without a closing dot - this is never a valid domain name).
 
+This plugin can only be used once per Server Block.
+
 ## Syntax
 
 ~~~

plugin/metrics/metrics.go

@@ -103,13 +103,8 @@ func (m *Metrics) OnStartup() error {
 	return nil
 }
 
-// OnShutdown tears down the metrics on shutdown and restart.
-func (m *Metrics) OnShutdown() error {
-	if m.ln != nil {
-		return m.ln.Close()
-	}
-	return nil
-}
+// OnShutdown tears down the metrics listener on shutdown and restart.
+func (m *Metrics) OnShutdown() error { return m.ln.Close() }
 
 func keys(m map[string]bool) []string {
 	sx := []string{}

plugin/metrics/setup.go

@@ -31,12 +31,12 @@ func setup(c *caddy.Controller) error {
 
 	for a, v := range uniqAddr.a {
 		if v == todo {
-			// During restarts we will keep this handler running, BUG.
 			c.OncePerServerBlock(m.OnStartup)
 		}
 		uniqAddr.a[a] = done
 	}
-	c.OnFinalShutdown(m.OnShutdown)
+
+	c.OnShutdown(m.OnShutdown)
 
 	return nil
 }
@@ -48,10 +48,12 @@ func prometheusParse(c *caddy.Controller) (*Metrics, error) {
 		uniqAddr.SetAddress(met.Addr)
 	}()
 
+	i := 0
 	for c.Next() {
-		if len(met.ZoneNames()) > 0 {
-			return met, c.Err("can only have one metrics module per server")
+		if i > 0 {
+			return nil, plugin.ErrOnce
 		}
+		i++
 
 		for _, z := range c.ServerBlockKeys {
 			met.AddZone(plugin.Host(z).Normalize())