[#1653] qos: Add metrics

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

internal/qos/stats.go

@@ -0,0 +1,28 @@
+package qos
+
+const unknownStatsTag = "unknown"
+
+var statTags = map[string]struct{}{
+	IOTagClient.String():     {},
+	IOTagBackground.String(): {},
+	IOTagInternal.String():   {},
+	IOTagPolicer.String():    {},
+	IOTagWritecache.String(): {},
+	IOTagCritical.String():   {},
+	unknownStatsTag:          {},
+}
+
+func createStats() map[string]*stat {
+	result := make(map[string]*stat)
+	for tag := range statTags {
+		result[tag] = &stat{}
+	}
+	return result
+}
+
+func getStat(tag string, stats map[string]*stat) *stat {
+	if v, ok := stats[tag]; ok {
+		return v
+	}
+	return stats[unknownStatsTag]
+}