feat: add HostFunc option to customize host metric label

options.go

@@ -124,6 +124,19 @@ func HandlerNameFunc(f func(c *gin.Context) string) PrometheusOption {
 	}
 }
 
+// HostFunc is an option allowing to set the HostFunc with New.
+// Use this option if you want to override the default behavior (i.e. using
+// c.Request.Host). This is useful to reduce metric cardinality when the Host
+// header varies (e.g. dynamic subdomains) but requests are handled the same.
+// Example:
+// r := gin.Default()
+// p := ginprom.New(HostFunc(func(c *gin.Context) string { return "my-service" }))
+func HostFunc(f func(c *gin.Context) string) PrometheusOption {
+	return func(p *Prometheus) {
+		p.HostFunc = f
+	}
+}
+
 // HandlerOpts is an option allowing to set the promhttp.HandlerOpts.
 // Use this option if you want to override the default zero value.
 func HandlerOpts(opts promhttp.HandlerOpts) PrometheusOption {

prom.go

@@ -21,6 +21,7 @@ var defaultNs = "gin"
 var defaultSys = "gonic"
 var defaultHandlerNameFunc = (*gin.Context).HandlerName
 var defaultRequestPathFunc = (*gin.Context).FullPath
+var defaultHostFunc = func(c *gin.Context) string { return c.Request.Host }
 
 var defaultReqCntMetricName = "requests_total"
 var defaultReqDurMetricName = "request_duration"
@@ -79,6 +80,7 @@ type Prometheus struct {
 	Registry        *prometheus.Registry
 	HandlerNameFunc func(c *gin.Context) string
 	RequestPathFunc func(c *gin.Context) string
+	HostFunc        func(c *gin.Context) string
 	HandlerOpts     promhttp.HandlerOpts
 
 	NativeHistogramBucketFactor     float64
@@ -272,6 +274,7 @@ func New(options ...PrometheusOption) *Prometheus {
 		Subsystem:                 defaultSys,
 		HandlerNameFunc:           defaultHandlerNameFunc,
 		RequestPathFunc:           defaultRequestPathFunc,
+		HostFunc:                  defaultHostFunc,
 		RequestCounterMetricName:  defaultReqCntMetricName,
 		RequestDurationMetricName: defaultReqDurMetricName,
 		RequestSizeMetricName:     defaultReqSzMetricName,
@@ -392,7 +395,8 @@ func (p *Prometheus) Instrument() gin.HandlerFunc {
 		elapsed := float64(time.Since(start)) / float64(time.Second)
 		resSz := float64(c.Writer.Size())
 
-		labels := []string{status, c.Request.Method, p.HandlerNameFunc(c), c.Request.Host, path}
+		host := p.HostFunc(c)
+		labels := []string{status, c.Request.Method, p.HandlerNameFunc(c), host, path}
 		if p.customCounterLabelsProvider != nil {
 			extraLabels := p.customCounterLabelsProvider(c)
 			for _, label := range p.customCounterLabels {
@@ -401,7 +405,7 @@ func (p *Prometheus) Instrument() gin.HandlerFunc {
 		}
 
 		p.reqCnt.WithLabelValues(labels...).Inc()
-		p.reqDur.WithLabelValues(c.Request.Method, path, c.Request.Host).Observe(elapsed)
+		p.reqDur.WithLabelValues(c.Request.Method, path, host).Observe(elapsed)
 		p.reqSz.Observe(float64(reqSz))
 		p.resSz.Observe(resSz)
 	}

prom_test.go

@@ -124,6 +124,38 @@ func TestHandlerNameFunc(t *testing.T) {
 	})
 }
 
+func TestHostFunc(t *testing.T) {
+	r := gin.New()
+	registry := prometheus.NewRegistry()
+	host := "normalized.example.com"
+	lhost := fmt.Sprintf("host=%q", host)
+
+	p := New(
+		HostFunc(func(c *gin.Context) string {
+			return host
+		}),
+		Registry(registry),
+		Engine(r),
+	)
+
+	r.Use(p.Instrument())
+
+	r.GET("/", func(context *gin.Context) {
+		context.Status(http.StatusOK)
+	})
+
+	g := gofight.New()
+
+	g.GET("/").Run(r, func(response gofight.HTTPResponse, request gofight.HTTPRequest) {
+		assert.Equal(t, response.Code, http.StatusOK)
+	})
+
+	g.GET(p.MetricsPath).Run(r, func(response gofight.HTTPResponse, request gofight.HTTPRequest) {
+		assert.Equal(t, response.Code, http.StatusOK)
+		assert.Contains(t, response.Body.String(), lhost)
+	})
+}
+
 func TestHandlerOpts(t *testing.T) {
 	r := gin.New()
 	registry := prometheus.NewRegistry()