Move static label normalization to constructor

lib/metrics/prometheus.js

@@ -14,6 +14,7 @@ class PrometheusMetric {
             if (Object.keys(this.staticLabels).length > 0) {
                 Object.keys(this.staticLabels).forEach((name) => {
                     options.labels.names.unshift(name);
+                    this.staticLabels[name] = this._normalize(this.staticLabels[name]);
                 });
             }
         }
@@ -32,9 +33,8 @@ class PrometheusMetric {
     _handleStaticLabels(labels) {
         if (this.staticLabels !== undefined) {
             Object.keys(this.staticLabels).forEach((name) => {
-                const normalizedLabel = this._normalize(this.staticLabels[name]);
-                if (labels.indexOf(normalizedLabel) === -1) {
-                    labels.unshift(normalizedLabel);
+                if (labels.indexOf(this.staticLabels[name]) === -1) {
+                    labels.unshift(this.staticLabels[name]);
                 }
             });
         }