Supply function hook to control request counter url label cardinality

README.md

@@ -31,4 +31,56 @@ func main() {
 }
 ```
 
-See the [example.go file](https://github.com/zsais/go-gin-prometheus/blob/master/example/example.go)
+See the [example.go file](https://github.com/zsais/go-gin-prometheus/blob/master/example/example.go)
+
+## Preserving a low cardinality for the request counter
+
+The request counter (`requests_total`) has a `url` label which,
+although desirable, can become problematic in cases where your
+application uses templated routes expecting a great number of
+variations, as Prometheus explicitly recommends against metrics having
+high cardinality dimensions:
+
+https://prometheus.io/docs/practices/naming/#labels
+
+If you have for instance a `/customer/:name` templated route and you
+don't want to generate a time series for every possible customer name,
+you could supply this mapping function to the middleware:
+
+```go
+package main
+
+import (
+	"github.com/gin-gonic/gin"
+	"github.com/zsais/go-gin-prometheus"
+)
+
+func main() {
+	r := gin.New()
+
+	p := ginprometheus.NewPrometheus("gin")
+
+	p.ReqCntURLLabelMappingFn = func(c *gin.Context) string {
+		url := c.Request.URL.String()
+		for _, p := range c.Params {
+			if p.Key == "name" {
+				url = strings.Replace(url, p.Value, ":name", 1)
+				break
+			}
+		}
+		return url
+	}
+
+	p.Use(r)
+
+	r.GET("/", func(c *gin.Context) {
+		c.JSON(200, "Hello world!")
+	})
+
+	r.Run(":29090")
+}
+```
+
+which would map `/customer/alice` and `/customer/bob` to their
+template `/customer/:name`, and thus preserve a low cardinality for
+our metrics.

middleware.go

@@ -16,6 +16,27 @@ import (
 
 var defaultMetricPath = "/metrics"
 
+/*
+RequestCounterURLLabelMappingFn is a function which can be supplied to the middleware to control
+the cardinality of the request counter's "url" label, which might be required in some contexts.
+For instance, if for a "/customer/:name" route you don't want to generate a time series for every
+possible customer name, you could use this function:
+
+func(c *gin.Context) string {
+	url := c.Request.URL.String()
+	for _, p := range c.Params {
+		if p.Key == "name" {
+			url = strings.Replace(url, p.Value, ":name", 1)
+			break
+		}
+	}
+	return url
+}
+
+which would map "/customer/alice" and "/customer/bob" to their template "/customer/:name".
+*/
+type RequestCounterURLLabelMappingFn func(c *gin.Context) string
+
 // Prometheus contains the metrics gathered by the instance and its path
 type Prometheus struct {
 	reqCnt               *prometheus.CounterVec
@@ -26,6 +47,8 @@ type Prometheus struct {
 	Ppg PrometheusPushGateway
 
 	MetricsPath string
+
+	ReqCntURLLabelMappingFn RequestCounterURLLabelMappingFn
 }
 
 // PrometheusPushGateway contains the configuration for pushing to a Prometheus pushgateway (optional)
@@ -51,6 +74,9 @@ func NewPrometheus(subsystem string) *Prometheus {
 
 	p := &Prometheus{
 		MetricsPath: defaultMetricPath,
+		ReqCntURLLabelMappingFn: func(c *gin.Context) string {
+			return c.Request.URL.String() // i.e. by default do nothing, i.e. return URL as is
+		},
 	}
 	p.registerMetrics(subsystem)
 
@@ -232,7 +258,8 @@ func (p *Prometheus) handlerFunc() gin.HandlerFunc {
 		resSz := float64(c.Writer.Size())
 
 		p.reqDur.Observe(elapsed)
-		p.reqCnt.WithLabelValues(status, c.Request.Method, c.HandlerName(), c.Request.Host, c.Request.URL.String()).Inc()
+		url := p.ReqCntURLLabelMappingFn(c)
+		p.reqCnt.WithLabelValues(status, c.Request.Method, c.HandlerName(), c.Request.Host, url).Inc()
 		p.reqSz.Observe(float64(reqSz))
 		p.resSz.Observe(resSz)
 	}