Fix missing tags for HTTP metrics (#538)

Author: mayurkale22

packages/opencensus-exporter-prometheus/src/prometheus-stats.ts

@@ -103,9 +103,16 @@ export class PrometheusStatsExporter implements StatsEventListener {
   private getLabelValues(columns: TagKey[], tags: Map<TagKey, TagValue>):
       labelValues {
     const labels: labelValues = {};
+
+    // TODO: Consider make original tags map with string:TagValue type.
+    const tagsWithStringKey: Map<string, TagValue|undefined> = new Map();
+    for (const key of tags.keys()) {
+      tagsWithStringKey.set(key.name, tags.get(key));
+    }
+
     columns.forEach((tagKey) => {
-      if (tags.has(tagKey)) {
-        const tagValue = tags.get(tagKey);
+      if (tagsWithStringKey.has(tagKey.name)) {
+        const tagValue = tagsWithStringKey.get(tagKey.name);
         if (tagValue) {
           labels[tagKey.name] = tagValue.value;
         }

packages/opencensus-exporter-prometheus/test/test-prometheus-stats.ts

@@ -24,7 +24,7 @@ describe('Prometheus Stats Exporter', () => {
   const prometheusServerUrl = `http://localhost:${options.port}/metrics`;
   const tagKey1 = {name: 'tagKey1'};
   const tagValue1 = {value: 'tagValue1'};
-  const tagKeys = [tagKey1];
+  const tagKeys = [{name: 'tagKey1'}];
   const tagMap = new TagMap();
   tagMap.set(tagKey1, tagValue1);