Allow custom labels to be added to net/http metrics (#306)

* Allow custom labels to be added to net/http metrics

* CHANGELOG entry

* Add license to labeler.go

* Address PR Feedback:

* Add mutex around access to labels
* Add Labeler use to handler example test

* Add getter to Labeler

CHANGELOG.md

@@ -12,6 +12,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 - Benchmark tests for the gRPC instrumentation. (#296)
 - Integration testing for the gRPC instrumentation. (#297)
+- Allow custom labels to be added to net/http metrics. (#306)
 
 ### Changed
 

instrumentation/net/http/handler.go

@@ -165,13 +165,16 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		},
 	})
 
+	labeler := &Labeler{}
+	ctx = injectLabeler(ctx, labeler)
+
 	h.handler.ServeHTTP(w, r.WithContext(ctx))
 
 	setAfterServeAttributes(span, bw.read, rww.written, rww.statusCode, bw.err, rww.err)
 
 	// Add request metrics
 
-	labels := semconv.HTTPServerMetricAttributesFromHTTPRequest(h.operation, r)
+	labels := append(labeler.Get(), semconv.HTTPServerMetricAttributesFromHTTPRequest(h.operation, r)...)
 
 	h.counters[RequestContentLength].Add(ctx, bw.read, labels...)
 	h.counters[ResponseContentLength].Add(ctx, rww.written, labels...)

instrumentation/net/http/handler_example_test.go

@@ -60,6 +60,8 @@ func ExampleNewHandler() {
 		otelhttp.WithRouteTag("/hello/:name", http.HandlerFunc(
 			func(w http.ResponseWriter, r *http.Request) {
 				ctx := r.Context()
+				labeler, _ := otelhttp.LabelerFromContext(ctx)
+
 				var name string
 				// Wrap another function in its own span
 				if err := func(ctx context.Context) error {
@@ -72,19 +74,22 @@ func ExampleNewHandler() {
 				}(ctx); err != nil {
 					log.Println("error figuring out name: ", err)
 					http.Error(w, err.Error(), http.StatusInternalServerError)
+					labeler.Add(label.Bool("error", true))
 					return
 				}
 
 				d, err := ioutil.ReadAll(r.Body)
 				if err != nil {
 					log.Println("error reading body: ", err)
 					w.WriteHeader(http.StatusBadRequest)
+					labeler.Add(label.Bool("error", true))
 					return
 				}
 
 				n, err := io.WriteString(w, "Hello, "+name+"!\nYou sent me this:\n"+string(d))
 				if err != nil {
 					log.Printf("error writing reply after %d bytes: %s", n, err)
+					labeler.Add(label.Bool("error", true))
 				}
 			}),
 		),

instrumentation/net/http/handler_test.go

@@ -50,6 +50,9 @@ func TestHandlerBasics(t *testing.T) {
 
 	h := NewHandler(
 		http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			l, _ := LabelerFromContext(r.Context())
+			l.Add(label.String("test", "label"))
+
 			if _, err := io.WriteString(w, "hello world"); err != nil {
 				t.Fatal(err)
 			}
@@ -73,6 +76,7 @@ func TestHandlerBasics(t *testing.T) {
 		semconv.HTTPSchemeHTTP,
 		semconv.HTTPHostKey.String(r.Host),
 		semconv.HTTPFlavorKey.String(fmt.Sprintf("1.%d", r.ProtoMinor)),
+		label.String("test", "label"),
 	}
 
 	assertMetricLabels(t, labelsToVerify, meterimpl.MeasurementBatches)

instrumentation/net/http/labeler.go

@@ -0,0 +1,65 @@
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package http
+
+import (
+	"context"
+	"sync"
+
+	"go.opentelemetry.io/otel/label"
+)
+
+// Labeler is used to allow instrumented HTTP handlers to add custom labels to
+// the metrics recorded by the net/http instrumentation.
+type Labeler struct {
+	mu     sync.Mutex
+	labels []label.KeyValue
+}
+
+// Add labels to a Labeler.
+func (l *Labeler) Add(ls ...label.KeyValue) {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	l.labels = append(l.labels, ls...)
+}
+
+// Labels returns a copy of the labels added to the Labeler.
+func (l *Labeler) Get() []label.KeyValue {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	ret := make([]label.KeyValue, len(l.labels))
+	copy(ret, l.labels)
+	return ret
+}
+
+type labelerContextKeyType int
+
+const lablelerContextKey labelerContextKeyType = 0
+
+func injectLabeler(ctx context.Context, l *Labeler) context.Context {
+	return context.WithValue(ctx, lablelerContextKey, l)
+}
+
+// LabelerFromContext retrieves a Labeler instance from the provided context if
+// one is available.  If no Labeler was found in the provided context a new, empty
+// Labeler is returned and the second return value is false.  In this case it is
+// safe to use the Labeler but any labels added to it will not be used.
+func LabelerFromContext(ctx context.Context) (*Labeler, bool) {
+	l, ok := ctx.Value(lablelerContextKey).(*Labeler)
+	if !ok {
+		l = &Labeler{}
+	}
+	return l, ok
+}