metrics: correctly register all metrics (#1335)

After initial startup, see if prometheus is loaded and if so, register
our metrics with it.
Stop doing the init() func and just use the sync.Once so we don't double
registrer our metrics.

plugin/autopath/metrics.go

@@ -18,12 +18,4 @@ var (
 	}, []string{})
 )
 
-// OnStartupMetrics sets up the metrics on startup.
-func OnStartupMetrics() error {
-	metricsOnce.Do(func() {
-		prometheus.MustRegister(AutoPathCount)
-	})
-	return nil
-}
-
-var metricsOnce sync.Once
+var once sync.Once