feat(go): expose gateway frame duration histogram

go/vehicle-gateway/internal/metrics/metrics.go

@@ -2,6 +2,7 @@ package metrics
 
 import (
 	"fmt"
+	"math"
 	"net/http"
 	"sort"
 	"strconv"
@@ -12,15 +13,23 @@ import (
 type Labels map[string]string
 
 type Registry struct {
-	mu       sync.RWMutex
-	counters map[string]map[string]float64
-	gauges   map[string]map[string]float64
+	mu         sync.RWMutex
+	counters   map[string]map[string]float64
+	gauges     map[string]map[string]float64
+	histograms map[string]*histogramFamily
+}
+
+type histogramFamily struct {
+	buckets map[string]map[float64]float64
+	counts  map[string]float64
+	sums    map[string]float64
 }
 
 func NewRegistry() *Registry {
 	return &Registry{
-		counters: map[string]map[string]float64{},
-		gauges:   map[string]map[string]float64{},
+		counters:   map[string]map[string]float64{},
+		gauges:     map[string]map[string]float64{},
+		histograms: map[string]*histogramFamily{},
 	}
 }
 
@@ -81,6 +90,40 @@ func (r *Registry) SetKafkaLag(name string, topic string, partition int, offset
 	}, float64(lag))
 }
 
+func (r *Registry) ObserveHistogram(name string, labels Labels, buckets []float64, value float64) {
+	name = strings.TrimSpace(name)
+	if r == nil || name == "" {
+		return
+	}
+	key := labelsKey(labels)
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	family := r.histograms[name]
+	if family == nil {
+		family = &histogramFamily{
+			buckets: map[string]map[float64]float64{},
+			counts:  map[string]float64{},
+			sums:    map[string]float64{},
+		}
+		r.histograms[name] = family
+	}
+	if family.buckets[key] == nil {
+		family.buckets[key] = map[float64]float64{}
+	}
+	normalizedBuckets := append([]float64(nil), buckets...)
+	sort.Float64s(normalizedBuckets)
+	for _, bucket := range normalizedBuckets {
+		if value <= bucket {
+			family.buckets[key][bucket]++
+		} else if _, ok := family.buckets[key][bucket]; !ok {
+			family.buckets[key][bucket] = 0
+		}
+	}
+	family.buckets[key][math.Inf(1)]++
+	family.counts[key]++
+	family.sums[key] += value
+}
+
 func (r *Registry) Render() string {
 	if r == nil {
 		return ""
@@ -107,6 +150,15 @@ func (r *Registry) Render() string {
 	for _, name := range names {
 		renderMetricFamily(&b, name, "gauge", r.gauges[name])
 	}
+
+	names = names[:0]
+	for name := range r.histograms {
+		names = append(names, name)
+	}
+	sort.Strings(names)
+	for _, name := range names {
+		renderHistogramFamily(&b, name, r.histograms[name])
+	}
 	return b.String()
 }
 
@@ -134,6 +186,72 @@ func renderMetricFamily(b *strings.Builder, name string, metricType string, valu
 	}
 }
 
+func renderHistogramFamily(b *strings.Builder, name string, family *histogramFamily) {
+	if family == nil {
+		return
+	}
+	b.WriteString("# TYPE ")
+	b.WriteString(name)
+	b.WriteString(" histogram\n")
+	var keys []string
+	for key := range family.counts {
+		keys = append(keys, key)
+	}
+	sort.Strings(keys)
+	for _, key := range keys {
+		var buckets []float64
+		for bucket := range family.buckets[key] {
+			buckets = append(buckets, bucket)
+		}
+		sort.Float64s(buckets)
+		for _, bucket := range buckets {
+			renderHistogramMetric(b, name+"_bucket", key, "le", bucketLabel(bucket), family.buckets[key][bucket])
+		}
+		renderMetricSample(b, name+"_count", key, family.counts[key])
+		renderMetricSample(b, name+"_sum", key, family.sums[key])
+	}
+}
+
+func renderHistogramMetric(b *strings.Builder, name string, key string, extraLabel string, extraValue string, value float64) {
+	b.WriteString(name)
+	labels := mergeLabelKey(key, extraLabel, extraValue)
+	if labels != "" {
+		b.WriteString("{")
+		b.WriteString(labels)
+		b.WriteString("}")
+	}
+	b.WriteString(" ")
+	b.WriteString(formatNumber(value))
+	b.WriteString("\n")
+}
+
+func renderMetricSample(b *strings.Builder, name string, key string, value float64) {
+	b.WriteString(name)
+	if key != "" {
+		b.WriteString("{")
+		b.WriteString(key)
+		b.WriteString("}")
+	}
+	b.WriteString(" ")
+	b.WriteString(formatNumber(value))
+	b.WriteString("\n")
+}
+
+func mergeLabelKey(key string, extraLabel string, extraValue string) string {
+	extra := fmt.Sprintf(`%s="%s"`, sanitizeLabelName(extraLabel), escapeLabelValue(extraValue))
+	if key == "" {
+		return extra
+	}
+	return extra + "," + key
+}
+
+func bucketLabel(value float64) string {
+	if math.IsInf(value, 1) {
+		return "+Inf"
+	}
+	return formatNumber(value)
+}
+
 func NewHandler(registry *Registry) http.Handler {
 	if registry == nil {
 		registry = NewRegistry()