Add DNS response time histogram metrics

- Add native exponential histogram for DNS response times
- Track response times by result type (filtered, not_filtered, etc.)
- Use factor 1.1 for automatic bucket generation
- Integrate with existing stats processing flow

Author: metalmatze

internal/metrics/dns.go

@@ -1,6 +1,8 @@
 package metrics
 
 import (
+	"time"
+
 	"github.com/prometheus/client_golang/prometheus"
 )
 
@@ -13,18 +15,31 @@ const (
 	ResultParental     = "parental"
 )
 
-// DNSQueriesByResult tracks DNS queries by their processing result
-var DNSQueriesByResult = prometheus.NewCounterVec(prometheus.CounterOpts{
-	Name: "adguard_dns_queries_by_result_total",
+// DNSQueries tracks DNS queries by their processing result
+var DNSQueries = prometheus.NewCounterVec(prometheus.CounterOpts{
+	Name: "adguard_dns_queries_total",
 	Help: "Total number of DNS queries by processing result",
 }, []string{"result"})
 
+// DNSResponseTime tracks DNS query response times using native exponential histogram
+var DNSResponseTime = prometheus.NewHistogramVec(prometheus.HistogramOpts{
+	Name:                        "adguard_dns_response_time_seconds",
+	Help:                        "DNS query response time in seconds",
+	NativeHistogramBucketFactor: 1.1,
+}, []string{"result"})
+
 // RegisterDNSMetrics registers all DNS-related metrics with the provided registry
 func RegisterDNSMetrics(registry *prometheus.Registry) {
-	registry.MustRegister(DNSQueriesByResult)
+	registry.MustRegister(DNSQueries)
+	registry.MustRegister(DNSResponseTime)
 }
 
 // IncrementDNSQueryByResult increments counters for a specific query result type
 func IncrementDNSQueryByResult(result string) {
-	DNSQueriesByResult.WithLabelValues(result).Inc()
+	DNSQueries.WithLabelValues(result).Inc()
+}
+
+// ObserveDNSResponseTime records a DNS query response time
+func ObserveDNSResponseTime(result string, duration time.Duration) {
+	DNSResponseTime.WithLabelValues(result).Observe(duration.Seconds())
 }

internal/stats/unit.go

@@ -9,12 +9,13 @@ import (
 	"slices"
 	"time"
 
-	"github.com/AdguardTeam/AdGuardHome/internal/aghnet"
-	"github.com/AdguardTeam/AdGuardHome/internal/metrics"
 	"github.com/AdguardTeam/dnsproxy/proxy"
 	"github.com/AdguardTeam/golibs/errors"
 	"github.com/AdguardTeam/golibs/logutil/slogutil"
 	"go.etcd.io/bbolt"
+
+	"github.com/AdguardTeam/AdGuardHome/internal/aghnet"
+	"github.com/AdguardTeam/AdGuardHome/internal/metrics"
 )
 
 const (
@@ -347,6 +348,7 @@ func (u *unit) add(e *Entry) {
 		resultLabel = "unknown"
 	}
 	metrics.IncrementDNSQueryByResult(resultLabel)
+	metrics.ObserveDNSResponseTime(resultLabel, e.ProcessingTime)
 
 	for _, s := range e.UpstreamStats {
 		if s.IsCached || s.Error != nil {