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/dnssec/handler.go

@@ -2,7 +2,6 @@ package dnssec
 
 import (
 	"context"
-	"sync"
 
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/plugin/metrics"
@@ -81,5 +80,3 @@ var (
 
 // Name implements the Handler interface.
 func (d Dnssec) Name() string { return "dnssec" }
-
-var once sync.Once