text: // Copyright 2018, OpenCensus 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 ochttp
  
  import (
  	"context"
  	"net/http"
  
  	"go.opencensus.io/tag"
  )
  
  // SetRoute sets the http_server_route tag to the given value.
  // It's useful when an HTTP framework does not support the http.Handler interface
  // and using WithRouteTag is not an option, but provides a way to hook into the request flow.
  func SetRoute(ctx context.Context, route string) {
  	if a, ok := ctx.Value(addedTagsKey{}).(*addedTags); ok {
  		a.t = append(a.t, tag.Upsert(KeyServerRoute, route))
  	}
  }
  
  // WithRouteTag returns an http.Handler that records stats with the
  // http_server_route tag set to the given value.
  func WithRouteTag(handler http.Handler, route string) http.Handler {
  	return taggedHandlerFunc(func(w http.ResponseWriter, r *http.Request) []tag.Mutator {
  		addRoute := []tag.Mutator{tag.Upsert(KeyServerRoute, route)}
  		ctx, _ := tag.New(r.Context(), addRoute...)
  		r = r.WithContext(ctx)
  		handler.ServeHTTP(w, r)
  		return addRoute
  	})
  }
  
  // taggedHandlerFunc is a http.Handler that returns tags describing the
  // processing of the request. These tags will be recorded along with the
  // measures in this package at the end of the request.
  type taggedHandlerFunc func(w http.ResponseWriter, r *http.Request) []tag.Mutator
  
  func (h taggedHandlerFunc) ServeHTTP(w http.ResponseWriter, r *http.Request) {
  	tags := h(w, r)
  	if a, ok := r.Context().Value(addedTagsKey{}).(*addedTags); ok {
  		a.t = append(a.t, tags...)
  	}
  }
  
  type addedTagsKey struct{}
  
  type addedTags struct {
  	t []tag.Mutator
  }
  