fn: API stats and tags reoorganization (#1171)

Make sure we can apply extra tags if RegisterAPIViews() is
provided with such tags. Deduplicate path/method/status and
always apply these default tags to appropriate views.

api/common/stats_utils.go

@@ -8,11 +8,16 @@ import (
 )
 
 func CreateView(measure stats.Measure, agg *view.Aggregation, tagKeys []string) *view.View {
+	keys := makeKeys(tagKeys)
+	return CreateViewWithTags(measure, agg, keys)
+}
+
+func CreateViewWithTags(measure stats.Measure, agg *view.Aggregation, tags []tag.Key) *view.View {
 	return &view.View{
 		Name:        measure.Name(),
 		Description: measure.Description(),
 		Measure:     measure,
-		TagKeys:     makeKeys(tagKeys),
+		TagKeys:     tags,
 		Aggregation: agg,
 	}
 }
@@ -21,14 +26,18 @@ func MakeMeasure(name string, desc string, unit string) *stats.Int64Measure {
 	return stats.Int64(name, desc, unit)
 }
 
+func MakeKey(name string) tag.Key {
+	key, err := tag.NewKey(name)
+	if err != nil {
+		logrus.WithError(err).Fatalf("Cannot create tag %s", name)
+	}
+	return key
+}
+
 func makeKeys(names []string) []tag.Key {
 	tagKeys := make([]tag.Key, len(names))
 	for i, name := range names {
-		key, err := tag.NewKey(name)
-		if err != nil {
-			logrus.Fatal(err)
-		}
-		tagKeys[i] = key
+		tagKeys[i] = MakeKey(name)
 	}
 	return tagKeys
 }