Replace . in metric names by _.

Because: "We are using a period to separate between the job name and the metric
name in shorthand; thusly, a metric ought not have a period in it." - Matt

exporter/gmond_collector.go

@@ -84,7 +84,7 @@ func (c *gmondCollector) Update() (updates int, err error) {
 		for _, host := range cluster.Hosts {
 
 			for _, metric := range host.Metrics {
-				name := strings.ToLower(metric.Name)
+				name := strings.Replace(strings.ToLower(metric.Name), ".", "_", -1)
 
 				var labels = map[string]string{
 					"hostname": host.Name,

exporter/munin_collector.go

@@ -169,7 +169,7 @@ func (c *muninCollector) registerMetrics() (err error) {
 		}
 
 		for metric, config := range configs {
-			metricName := name + "-" + metric
+			metricName := strings.Replace(name+"-"+metric, ".", "_", -1)
 			desc := graphConfig["graph_title"] + ": " + config["label"]
 			if config["info"] != "" {
 				desc = desc + ", " + config["info"]