Merge pull request #2970 from dims/revert-metrics-append

Revert "Remove unused MetricSet.Append() method"

Author: bobbypage

container/factory.go

@@ -145,6 +145,16 @@ func (ms MetricSet) Difference(ms1 MetricSet) MetricSet {
 	return result
 }
 
+func (ms MetricSet) Append(ms1 MetricSet) MetricSet {
+	result := ms
+	for kind := range ms1 {
+		if !ms.Has(kind) {
+			result.add(kind)
+		}
+	}
+	return result
+}
+
 // All registered auth provider plugins.
 var pluginsLock sync.Mutex
 var plugins = make(map[string]Plugin)