New feature: prometheus metrics endpoint (#46)

internal/metrics/metrics.go

@@ -0,0 +1,122 @@
+package metrics
+
+import (
+	"fmt"
+	"sync"
+	"time"
+
+	"github.com/gin-gonic/gin"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+var (
+	enabled          bool
+	initOnce         sync.Once
+	requestsTotal    *prometheus.CounterVec
+	requestDuration  *prometheus.HistogramVec
+	requestsInFlight prometheus.Gauge
+	geoLookups       *prometheus.CounterVec
+	portScans        prometheus.Counter
+	dnsQueries       *prometheus.CounterVec
+)
+
+func Enable() {
+	initOnce.Do(func() {
+		enabled = true
+
+		requestsTotal = promauto.NewCounterVec(
+			prometheus.CounterOpts{
+				Name: "whatismyip_http_requests_total",
+				Help: "Total number of HTTP requests",
+			},
+			[]string{"method", "path", "status"},
+		)
+
+		requestDuration = promauto.NewHistogramVec(
+			prometheus.HistogramOpts{
+				Name:    "whatismyip_http_request_duration_seconds",
+				Help:    "HTTP request latency in seconds",
+				Buckets: prometheus.DefBuckets,
+			},
+			[]string{"method", "path"},
+		)
+
+		requestsInFlight = promauto.NewGauge(
+			prometheus.GaugeOpts{
+				Name: "whatismyip_http_requests_in_flight",
+				Help: "Current number of HTTP requests being processed",
+			},
+		)
+
+		geoLookups = promauto.NewCounterVec(
+			prometheus.CounterOpts{
+				Name: "whatismyip_geo_lookups_total",
+				Help: "Total number of geo lookups",
+			},
+			[]string{"type"},
+		)
+
+		portScans = promauto.NewCounter(
+			prometheus.CounterOpts{
+				Name: "whatismyip_port_scans_total",
+				Help: "Total number of port scan requests",
+			},
+		)
+
+		dnsQueries = promauto.NewCounterVec(
+			prometheus.CounterOpts{
+				Name: "whatismyip_dns_queries_total",
+				Help: "Total number of DNS queries",
+			},
+			[]string{"query_type", "rcode"},
+		)
+	})
+}
+
+func GinMiddleware() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		if !enabled {
+			c.Next()
+			return
+		}
+
+		start := time.Now()
+		path := c.FullPath()
+		if path == "" {
+			path = "/404" // group 404s
+		}
+
+		requestsInFlight.Inc()
+		defer requestsInFlight.Dec()
+
+		c.Next()
+
+		duration := time.Since(start).Seconds()
+		status := c.Writer.Status()
+
+		requestsTotal.WithLabelValues(c.Request.Method, path, fmt.Sprintf("%dxx", status/100)).Inc()
+		requestDuration.WithLabelValues(c.Request.Method, path).Observe(duration)
+	}
+}
+
+func RecordGeoLookup(lookupType string) {
+	if !enabled {
+		return
+	}
+	geoLookups.WithLabelValues(lookupType).Inc()
+}
+
+func RecordPortScan() {
+	if !enabled {
+		return
+	}
+	portScans.Inc()
+}
+
+func RecordDNSQuery(queryType string, rcode string) {
+	if !enabled {
+		return
+	}
+	dnsQueries.WithLabelValues(queryType, rcode).Inc()
+}