Avoid allocation when counting tag keys

A new sorted slice was called by the monitor func every 10s.  The
tag keys don't need to be sorted so this avoid the allocation of the
slice and one during sorting.

tsdb/meta.go

@@ -1893,6 +1893,17 @@ func MarshalTags(tags map[string]string) []byte {
 	return b
 }
 
+// WalkTagKeys calls fn for each tag key associated with m.  The order of the
+// keys is undefined.
+func (m *Measurement) WalkTagKeys(fn func(k string)) {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+
+	for k := range m.seriesByTagKeyValue {
+		fn(k)
+	}
+}
+
 // TagKeys returns a list of the measurement's tag names.
 func (m *Measurement) TagKeys() []string {
 	m.mu.RLock()

tsdb/shard.go

@@ -853,7 +853,7 @@ func (s *Shard) monitor() {
 			}
 
 			for _, m := range s.index.Measurements() {
-				for _, k := range m.TagKeys() {
+				m.WalkTagKeys(func(k string) {
 					n := m.Cardinality(k)
 					perc := int(float64(n) / float64(s.options.Config.MaxValuesPerTag) * 100)
 					if perc > 100 {
@@ -865,7 +865,7 @@ func (s *Shard) monitor() {
 						s.logger.Printf("WARN: %d%% of max-values-per-tag limit exceeded: (%d/%d), db=%s shard=%d measurement=%s tag=%s",
 							perc, n, s.options.Config.MaxValuesPerTag, s.database, s.id, m.Name, k)
 					}
-				}
+				})
 			}
 		}
 	}