APM: Reduce number of allocs from sampler metrics (#33417)

Author: ajgajg1134

pkg/trace/sampler/metrics.go

@@ -24,28 +24,33 @@ type metrics struct {
 	value      map[metricsKey]metricsValue
 }
 
-type metricsKey [3]string
+type metricsKey struct {
+	targetService    string
+	targetEnv        string
+	samplingPriority string
+}
 
 func newMetricsKey(service, env string, samplingPriority *SamplingPriority) metricsKey {
-	var key metricsKey
-	if service != "" {
-		key[0] = "target_service:" + service
-	}
-	if env != "" {
-		key[1] = "target_env:" + env
+	mk := metricsKey{
+		targetService: service,
+		targetEnv:     env,
 	}
 	if samplingPriority != nil {
-		key[2] = samplingPriority.tag()
+		mk.samplingPriority = samplingPriority.tagValue()
 	}
-	return key
+	return mk
 }
 
 func (k metricsKey) tags() []string {
-	tags := make([]string, 0, len(k))
-	for _, v := range k {
-		if v != "" {
-			tags = append(tags, v)
-		}
+	tags := make([]string, 0, 3) // Pre-allocate number of fields for efficiency
+	if k.targetService != "" {
+		tags = append(tags, "target_service:"+k.targetService)
+	}
+	if k.targetEnv != "" {
+		tags = append(tags, "target_env:"+k.targetEnv)
+	}
+	if k.samplingPriority != "" {
+		tags = append(tags, "sampling_priority:"+k.samplingPriority)
 	}
 	return tags
 }

pkg/trace/sampler/prioritysampler_test.go

@@ -10,13 +10,14 @@ import (
 	"testing"
 	"time"
 
+	"github.com/golang/mock/gomock"
+	"github.com/stretchr/testify/assert"
+	"go.uber.org/atomic"
+
 	pb "github.com/DataDog/datadog-agent/pkg/proto/pbgo/trace"
 	"github.com/DataDog/datadog-agent/pkg/trace/config"
 	"github.com/DataDog/datadog-go/v5/statsd"
 	mockStatsd "github.com/DataDog/datadog-go/v5/statsd/mocks"
-	"github.com/golang/mock/gomock"
-	"github.com/stretchr/testify/assert"
-	"go.uber.org/atomic"
 )
 
 func randomTraceID() uint64 {
@@ -89,7 +90,7 @@ func TestPrioritySample(t *testing.T) {
 		},
 	}
 	for _, tt := range tests {
-		t.Run(tt.priority.tag(), func(t *testing.T) {
+		t.Run(tt.priority.tagValue(), func(t *testing.T) {
 			ctrl := gomock.NewController(t)
 			defer ctrl.Finish()
 			statsdClient := mockStatsd.NewMockClientInterface(ctrl)
@@ -107,7 +108,7 @@ func TestPrioritySample(t *testing.T) {
 			if tt.priority == PriorityNone {
 				expectedTagsA = append(expectedTagsA, "sampling_priority:auto_drop")
 			} else {
-				expectedTagsA = append(expectedTagsA, tt.priority.tag())
+				expectedTagsA = append(expectedTagsA, "sampling_priority:"+tt.priority.tagValue())
 			}
 			chunkA, rootA := getTestTraceWithService("service-a", s)
 			chunkA.Priority = int32(tt.priority)
@@ -121,7 +122,7 @@ func TestPrioritySample(t *testing.T) {
 			if tt.priority == PriorityNone {
 				expectedTagsB = append(expectedTagsB, "sampling_priority:auto_drop")
 			} else {
-				expectedTagsB = append(expectedTagsB, tt.priority.tag())
+				expectedTagsB = append(expectedTagsB, "sampling_priority:"+tt.priority.tagValue())
 			}
 			chunkB, rootB := getTestTraceWithService("service-b", s)
 			chunkB.Priority = int32(tt.priority)

pkg/trace/sampler/sampler.go

@@ -68,21 +68,19 @@ const (
 	samplerHasher = uint64(1111111111111111111)
 )
 
-func (s SamplingPriority) tag() string {
-	var v string
+func (s SamplingPriority) tagValue() string {
 	switch s {
 	case PriorityUserDrop:
-		v = "manual_drop"
+		return "manual_drop"
 	case PriorityAutoDrop:
-		v = "auto_drop"
+		return "auto_drop"
 	case PriorityAutoKeep:
-		v = "auto_keep"
+		return "auto_keep"
 	case PriorityUserKeep:
-		v = "manual_keep"
+		return "manual_keep"
 	default:
-		v = "none"
+		return "none"
 	}
-	return "sampling_priority:" + v
 }
 
 // SampleByRate returns whether to keep a trace, based on its ID and a sampling rate.