address some comments

src/cmd/services/m3coordinator/downsample/metrics_appender.go

@@ -47,14 +47,15 @@ import (
 )
 
 var (
-	m3TypeTag      = []byte("__m3_type__")
 	m3CounterValue = []byte("counter")
 	m3GaugeValue   = []byte("gauge")
 	m3TimerValue   = []byte("timer")
 
-	m3MetricsPrefix              = []byte("__m3")
-	m3MetricsPrefixString        = string(m3MetricsPrefix)
-	m3MetricsGraphiteAggregation = []byte("__m3_graphite_aggregation__")
+	m3MetricsPrefix       = []byte("__m3")
+	m3MetricsPrefixString = string(m3MetricsPrefix)
+
+	m3TypeTag                    = []byte(m3MetricsPrefixString + "_type__")
+	m3MetricsGraphiteAggregation = []byte(m3MetricsPrefixString + "_graphite_aggregation__")
 )
 
 type metricsAppenderPool struct {
@@ -121,13 +122,8 @@ func newMetricsAppender(pool *metricsAppenderPool) *metricsAppender {
 func (a *metricsAppender) reset(opts metricsAppenderOptions) {
 	a.metricsAppenderOptions = opts
 
-	// Copy over any previous inuse encoders to the cached encoders list. If
-	// there are no cached encoders then create one.
-	a.cachedEncoders = append(a.cachedEncoders, a.inuseEncoders...)
-	a.inuseEncoders = a.inuseEncoders[:0]
-	if len(a.cachedEncoders) == 0 {
-		a.cachedEncoders = append(a.cachedEncoders, opts.tagEncoderPool.Get())
-	}
+	// Copy over any previous inuse encoders to the cached encoders list.
+	a.moveInuseEncoders()
 
 	// Make sure a.defaultStagedMetadatasCopies is right length.
 	capRequired := len(opts.defaultStagedMetadatasProtos)
@@ -405,15 +401,22 @@ func (a *metricsAppender) NextMetric() {
 	a.tags.values = a.tags.values[:0]
 
 	// Move the inuse encoders to cached as we should be done with using them.
-	a.cachedEncoders = append(a.cachedEncoders, a.inuseEncoders...)
-	a.inuseEncoders = a.inuseEncoders[:0]
+	a.moveInuseEncoders()
 }
 
 func (a *metricsAppender) Finalize() {
 	// Return to pool.
 	a.pool.Put(a)
 }
 
+func (a *metricsAppender) moveInuseEncoders() {
+	a.cachedEncoders = append(a.cachedEncoders, a.inuseEncoders...)
+	for i := range a.inuseEncoders {
+		a.inuseEncoders[i] = nil
+	}
+	a.inuseEncoders = a.inuseEncoders[:0]
+}
+
 func (a *metricsAppender) tagEncoder() serialize.TagEncoder {
 	// Take an encoder from the cached encoder list, if not present get one
 	// from the pool. Add the returned encoder to the used list.
@@ -497,7 +500,6 @@ func (a *metricsAppender) newSamplesAppender(
 	tags *tags,
 	sm metadata.StagedMetadata,
 ) (samplesAppender, error) {
-	// Get a new tag encoder from the unused list if not present create a new one.
 	tagEncoder := a.tagEncoder()
 	if err := tagEncoder.Encode(tags); err != nil {
 		return samplesAppender{}, err
@@ -549,7 +551,7 @@ func augmentTags(
 
 	// Add any additional tags we need to.
 	for _, tag := range t {
-		// If these are simply random tags to be added, then just add them.
+		// If the tag is not special tag, then just add it.
 		if !bytes.HasPrefix(tag.Name, m3MetricsPrefix) {
 			if len(tag.Name) > 0 && len(tag.Value) > 0 {
 				tags.append(tag.Name, tag.Value)

src/cmd/services/m3coordinator/downsample/options.go

@@ -319,10 +319,10 @@ type Tag struct {
 
 // Rule returns the mapping rule for the mapping rule configuration.
 func (r MappingRuleConfiguration) Rule() (view.MappingRule, error) {
-	ruleID := uuid.New()
+	id := uuid.New()
 	name := r.Name
 	if name == "" {
-		name = ruleID
+		name = id
 	}
 	filter := r.Filter
 
@@ -350,7 +350,7 @@ func (r MappingRuleConfiguration) Rule() (view.MappingRule, error) {
 	}
 
 	return view.MappingRule{
-		ID:              ruleID,
+		ID:              id,
 		Name:            name,
 		Filter:          filter,
 		AggregationID:   aggID,
@@ -696,6 +696,9 @@ func (cfg Configuration) newAggregator(o DownsamplerOptions) (agg, error) {
 		}
 
 		for _, rollupRule := range cfg.Rules.RollupRules {
+			if strings.Contains(rollupRule.Filter, m3MetricsPrefixString) {
+				m3PrefixFilter = true
+			}
 			rule, err := rollupRule.Rule()
 			if err != nil {
 				return agg{}, err

src/query/server/query.go

@@ -629,11 +629,7 @@ func newM3DBStorage(
 	if n := namespaces.NumAggregatedClusterNamespaces(); n > 0 {
 		logger.Info("configuring downsampler to use with aggregated cluster namespaces",
 			zap.Int("numAggregatedClusterNamespaces", n))
-		var (
-			autoMappingRules []downsample.AutoMappingRule
-			err              error
-		)
-		autoMappingRules, err = newDownsamplerAutoMappingRules(namespaces)
+		autoMappingRules, err := newDownsamplerAutoMappingRules(namespaces)
 		if err != nil {
 			return nil, nil, nil, nil, err
 		}