middleware/metrics: survive restart (#542)

* middleware/metrics: survive restart

Keep the handler running during restart. Stopping and starting the
handler results in "address in use" - sometimes, meaning the reload
will be flaky. In turn this behavior means any changes to the monitor
stanza are not picked up.

* remove resync

middleware/metrics/setup.go

@@ -28,9 +28,10 @@ func setup(c *caddy.Controller) error {
 		return m
 	})
 
+	// During restarts we will keep this handler running.
 	metricsOnce.Do(func() {
-		c.OnStartup(m.OnStartup)
-		c.OnShutdown(m.OnShutdown)
+		c.OncePerServerBlock(m.OnStartup)
+		c.OnFinalShutdown(m.OnShutdown)
 	})
 
 	return nil