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/cache/setup.go

@@ -34,11 +34,9 @@ func setup(c *caddy.Controller) error {
 	})
 
 	c.OnStartup(func() error {
-		once.Do(func() {
-			metrics.MustRegister(c,
-				cacheSize, cacheHits, cacheMisses,
-				cachePrefetches, cacheDrops)
-		})
+		metrics.MustRegister(c,
+			cacheSize, cacheHits, cacheMisses,
+			cachePrefetches, cacheDrops)
 		return nil
 	})