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/health/setup.go

@@ -55,7 +55,7 @@ func setup(c *caddy.Controller) error {
 	})
 
 	c.OnStartup(func() error {
-		once.Do(func() { metrics.MustRegister(c, HealthDuration) })
+		metrics.MustRegister(c, HealthDuration)
 		return nil
 	})