Ensure Re-register of metrics variables after a reload (#2080)

* - ensure plugins that use prometheus.MustRegister, re-register after reload
- removing once.Do on the startup function was simplest way to do it.

* - fix underscored names (advice of bot)

* - tune existing UT for reload, and add a test verifying failing reload does not prevent correct registering for metrics

* - ensure different ports for tests that can run in same time ..

plugin/forward/metrics.go

@@ -1,8 +1,6 @@
 package forward
 
 import (
-	"sync"
-
 	"github.com/coredns/coredns/plugin"
 
 	"github.com/prometheus/client_golang/prometheus"
@@ -48,5 +46,3 @@ var (
 		Help:      "Gauge of open sockets per upstream.",
 	}, []string{"to"})
 )
-
-var once sync.Once