plugin/metrics: add MustRegister function (#1648)

This registers the Collectors iff the metrics plugin has been loaded.
Safes a bunch of code in each and every plugin's setup code.

plugin/cache/setup.go

@@ -32,18 +32,10 @@ func setup(c *caddy.Controller) error {
 
 	c.OnStartup(func() error {
 		once.Do(func() {
-			m := dnsserver.GetConfig(c).Handler("prometheus")
-			if m == nil {
-				return
-			}
-			if x, ok := m.(*metrics.Metrics); ok {
-				x.MustRegister(cacheSize)
-				x.MustRegister(cacheCapacity)
-				x.MustRegister(cacheHits)
-				x.MustRegister(cacheMisses)
-				x.MustRegister(cachePrefetches)
-				x.MustRegister(cacheDrops)
-			}
+			metrics.MustRegister(c,
+				cacheSize, cacheCapacity,
+				cacheHits, cacheMisses,
+				cachePrefetches, cacheDrops)
 		})
 		return nil
 	})