promhttp: implement WithXFromContext in terms of WithXFromRequest

This change adds WithLabelFromRequest and WithExemplarFromRequest
options and updates FromContext counterparts to be a convenience
wrappers for these options.

For example, without this change, setting a label based on
http.Request.Pattern requires some juggling with context:

	var ctxHTTPRequestKey = httpRequestContext{}

	type httpRequestContext struct {
		*http.Request
	}

	func httpPatternFromContext(ctx context.Context) string {
		r := ctx.Value(ctxHTTPRequestKey).(*httpRequestContext)
		return r.Pattern
	}

	func instrumentHTTPHandler(h http.Handler) http.Handler {
		h = promhttp.InstrumentHandlerCounter(httpRequestsTotal, h,
			promhttp.WithLabelFromCtx("handler", httpPatternFromContext),
		)
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			var c httpRequestContext
			ctx := context.WithValue(r.Context(), ctxHTTPRequestKey, &c)
			r = r.WithContext(ctx)
			c = httpRequestContext{r}
			h.ServeHTTP(w, r)
		})
	}

promhttp.WithLabelFromRequest allows to access http.Request directly:

	func instrumentHTTPHandler(h http.Handler) http.Handler {
		return promhttp.InstrumentHandlerCounter(httpRequestsTotal, h,
			promhttp.WithLabelFromRequest("handler", func(r *http.Request) string {
				return r.Pattern
			}),
		)
	}

Author: tie

prometheus/promhttp/instrument_client.go

@@ -75,10 +75,10 @@ func InstrumentRoundTripperCounter(counter *prometheus.CounterVec, next http.Rou
 		resp, err := next.RoundTrip(r)
 		if err == nil {
 			l := labels(code, method, r.Method, resp.StatusCode, rtOpts.extraMethods...)
-			for label, resolve := range rtOpts.extraLabelsFromCtx {
-				l[label] = resolve(resp.Request.Context())
+			for label, resolve := range rtOpts.extraLabelsFromRequest {
+				l[label] = resolve(resp.Request)
 			}
-			addWithExemplar(counter.With(l), 1, rtOpts.getExemplarFn(r.Context()))
+			addWithExemplar(counter.With(l), 1, rtOpts.getExemplarFn(r))
 		}
 		return resp, err
 	}
@@ -119,10 +119,10 @@ func InstrumentRoundTripperDuration(obs prometheus.ObserverVec, next http.RoundT
 		resp, err := next.RoundTrip(r)
 		if err == nil {
 			l := labels(code, method, r.Method, resp.StatusCode, rtOpts.extraMethods...)
-			for label, resolve := range rtOpts.extraLabelsFromCtx {
-				l[label] = resolve(resp.Request.Context())
+			for label, resolve := range rtOpts.extraLabelsFromRequest {
+				l[label] = resolve(resp.Request)
 			}
-			observeWithExemplar(obs.With(l), time.Since(start).Seconds(), rtOpts.getExemplarFn(r.Context()))
+			observeWithExemplar(obs.With(l), time.Since(start).Seconds(), rtOpts.getExemplarFn(r))
 		}
 		return resp, err
 	}

prometheus/promhttp/instrument_server.go

@@ -97,21 +97,21 @@ func InstrumentHandlerDuration(obs prometheus.ObserverVec, next http.Handler, op
 			next.ServeHTTP(d, r)
 
 			l := labels(code, method, r.Method, d.Status(), hOpts.extraMethods...)
-			for label, resolve := range hOpts.extraLabelsFromCtx {
-				l[label] = resolve(r.Context())
+			for label, resolve := range hOpts.extraLabelsFromRequest {
+				l[label] = resolve(r)
 			}
-			observeWithExemplar(obs.With(l), time.Since(now).Seconds(), hOpts.getExemplarFn(r.Context()))
+			observeWithExemplar(obs.With(l), time.Since(now).Seconds(), hOpts.getExemplarFn(r))
 		}
 	}
 
 	return func(w http.ResponseWriter, r *http.Request) {
 		now := time.Now()
 		next.ServeHTTP(w, r)
 		l := labels(code, method, r.Method, 0, hOpts.extraMethods...)
-		for label, resolve := range hOpts.extraLabelsFromCtx {
-			l[label] = resolve(r.Context())
+		for label, resolve := range hOpts.extraLabelsFromRequest {
+			l[label] = resolve(r)
 		}
-		observeWithExemplar(obs.With(l), time.Since(now).Seconds(), hOpts.getExemplarFn(r.Context()))
+		observeWithExemplar(obs.With(l), time.Since(now).Seconds(), hOpts.getExemplarFn(r))
 	}
 }
 
@@ -147,21 +147,21 @@ func InstrumentHandlerCounter(counter *prometheus.CounterVec, next http.Handler,
 			next.ServeHTTP(d, r)
 
 			l := labels(code, method, r.Method, d.Status(), hOpts.extraMethods...)
-			for label, resolve := range hOpts.extraLabelsFromCtx {
-				l[label] = resolve(r.Context())
+			for label, resolve := range hOpts.extraLabelsFromRequest {
+				l[label] = resolve(r)
 			}
-			addWithExemplar(counter.With(l), 1, hOpts.getExemplarFn(r.Context()))
+			addWithExemplar(counter.With(l), 1, hOpts.getExemplarFn(r))
 		}
 	}
 
 	return func(w http.ResponseWriter, r *http.Request) {
 		next.ServeHTTP(w, r)
 
 		l := labels(code, method, r.Method, 0, hOpts.extraMethods...)
-		for label, resolve := range hOpts.extraLabelsFromCtx {
-			l[label] = resolve(r.Context())
+		for label, resolve := range hOpts.extraLabelsFromRequest {
+			l[label] = resolve(r)
 		}
-		addWithExemplar(counter.With(l), 1, hOpts.getExemplarFn(r.Context()))
+		addWithExemplar(counter.With(l), 1, hOpts.getExemplarFn(r))
 	}
 }
 
@@ -200,10 +200,10 @@ func InstrumentHandlerTimeToWriteHeader(obs prometheus.ObserverVec, next http.Ha
 		now := time.Now()
 		d := newDelegator(w, func(status int) {
 			l := labels(code, method, r.Method, status, hOpts.extraMethods...)
-			for label, resolve := range hOpts.extraLabelsFromCtx {
-				l[label] = resolve(r.Context())
+			for label, resolve := range hOpts.extraLabelsFromRequest {
+				l[label] = resolve(r)
 			}
-			observeWithExemplar(obs.With(l), time.Since(now).Seconds(), hOpts.getExemplarFn(r.Context()))
+			observeWithExemplar(obs.With(l), time.Since(now).Seconds(), hOpts.getExemplarFn(r))
 		})
 		next.ServeHTTP(d, r)
 	}
@@ -244,10 +244,10 @@ func InstrumentHandlerRequestSize(obs prometheus.ObserverVec, next http.Handler,
 			size := computeApproximateRequestSize(r)
 
 			l := labels(code, method, r.Method, d.Status(), hOpts.extraMethods...)
-			for label, resolve := range hOpts.extraLabelsFromCtx {
-				l[label] = resolve(r.Context())
+			for label, resolve := range hOpts.extraLabelsFromRequest {
+				l[label] = resolve(r)
 			}
-			observeWithExemplar(obs.With(l), float64(size), hOpts.getExemplarFn(r.Context()))
+			observeWithExemplar(obs.With(l), float64(size), hOpts.getExemplarFn(r))
 		}
 	}
 
@@ -256,10 +256,10 @@ func InstrumentHandlerRequestSize(obs prometheus.ObserverVec, next http.Handler,
 		size := computeApproximateRequestSize(r)
 
 		l := labels(code, method, r.Method, 0, hOpts.extraMethods...)
-		for label, resolve := range hOpts.extraLabelsFromCtx {
-			l[label] = resolve(r.Context())
+		for label, resolve := range hOpts.extraLabelsFromRequest {
+			l[label] = resolve(r)
 		}
-		observeWithExemplar(obs.With(l), float64(size), hOpts.getExemplarFn(r.Context()))
+		observeWithExemplar(obs.With(l), float64(size), hOpts.getExemplarFn(r))
 	}
 }
 
@@ -296,10 +296,10 @@ func InstrumentHandlerResponseSize(obs prometheus.ObserverVec, next http.Handler
 		next.ServeHTTP(d, r)
 
 		l := labels(code, method, r.Method, d.Status(), hOpts.extraMethods...)
-		for label, resolve := range hOpts.extraLabelsFromCtx {
-			l[label] = resolve(r.Context())
+		for label, resolve := range hOpts.extraLabelsFromRequest {
+			l[label] = resolve(r)
 		}
-		observeWithExemplar(obs.With(l), float64(d.Written()), hOpts.getExemplarFn(r.Context()))
+		observeWithExemplar(obs.With(l), float64(d.Written()), hOpts.getExemplarFn(r))
 	})
 }
 

prometheus/promhttp/option.go

@@ -15,6 +15,7 @@ package promhttp
 
 import (
 	"context"
+	"net/http"
 
 	"github.com/prometheus/client_golang/prometheus"
 )
@@ -24,28 +25,31 @@ type Option interface {
 	apply(*options)
 }
 
+// LabelValueFromRequest is used to compute the label value from request.
+type LabelValueFromRequest func(request *http.Request) string
+
 // LabelValueFromCtx are used to compute the label value from request context.
 // Context can be filled with values from request through middleware.
 type LabelValueFromCtx func(ctx context.Context) string
 
 // options store options for both a handler or round tripper.
 type options struct {
-	extraMethods       []string
-	getExemplarFn      func(requestCtx context.Context) prometheus.Labels
-	extraLabelsFromCtx map[string]LabelValueFromCtx
+	extraMethods           []string
+	getExemplarFn          func(req *http.Request) prometheus.Labels
+	extraLabelsFromRequest map[string]LabelValueFromRequest
 }
 
 func defaultOptions() *options {
 	return &options{
-		getExemplarFn:      func(ctx context.Context) prometheus.Labels { return nil },
-		extraLabelsFromCtx: map[string]LabelValueFromCtx{},
+		getExemplarFn:          func(req *http.Request) prometheus.Labels { return nil },
+		extraLabelsFromRequest: map[string]LabelValueFromRequest{},
 	}
 }
 
 func (o *options) emptyDynamicLabels() prometheus.Labels {
 	labels := prometheus.Labels{}
 
-	for label := range o.extraLabelsFromCtx {
+	for label := range o.extraLabelsFromRequest {
 		labels[label] = ""
 	}
 
@@ -66,19 +70,39 @@ func WithExtraMethods(methods ...string) Option {
 	})
 }
 
+// WithExemplarFromRequest allows to inject function that will get exemplar from request that will be put to counter and histogram metrics.
+// If the function returns nil labels or the metric does not support exemplars, no exemplar will be added (noop), but
+// metric will continue to observe/increment.
+func WithExemplarFromRequest(getExemplarFn func(req *http.Request) prometheus.Labels) Option {
+	return optionApplyFunc(func(o *options) {
+		o.getExemplarFn = getExemplarFn
+	})
+}
+
 // WithExemplarFromContext allows to inject function that will get exemplar from context that will be put to counter and histogram metrics.
 // If the function returns nil labels or the metric does not support exemplars, no exemplar will be added (noop), but
 // metric will continue to observe/increment.
 func WithExemplarFromContext(getExemplarFn func(requestCtx context.Context) prometheus.Labels) Option {
 	return optionApplyFunc(func(o *options) {
-		o.getExemplarFn = getExemplarFn
+		o.getExemplarFn = func(req *http.Request) prometheus.Labels {
+			return getExemplarFn(req.Context())
+		}
+	})
+}
+
+// WithLabelFromRequest registers a label for dynamic resolution with access to the request.
+func WithLabelFromRequest(name string, valueFn LabelValueFromRequest) Option {
+	return optionApplyFunc(func(o *options) {
+		o.extraLabelsFromRequest[name] = valueFn
 	})
 }
 
 // WithLabelFromCtx registers a label for dynamic resolution with access to context.
 // See the example for ExampleInstrumentHandlerWithLabelResolver for example usage
 func WithLabelFromCtx(name string, valueFn LabelValueFromCtx) Option {
 	return optionApplyFunc(func(o *options) {
-		o.extraLabelsFromCtx[name] = valueFn
+		o.extraLabelsFromRequest[name] = func(req *http.Request) string {
+			return valueFn(req.Context())
+		}
 	})
 }