ocgrpc server instrumentation drops stats tags from the passed OC context

[parabolala]

Please answer these questions before submitting a bug report.

What version of OpenCensus are you using?

v0.22.5

What version of Go are you using?

1.15

What did you do?

I wanted to add an extra label/tag to the ocgcrpc-generated server metrics: client ID, and export them with a custom view. See a sample gRPC+metrics server. In particular lines around clientIDValue.

package main

import (
	"flag"
	"fmt"
	"log"
	"net/http"
	"strings"

	"contrib.go.opencensus.io/exporter/prometheus"
	"github.com/golang/glog"
	"go.opencensus.io/plugin/ocgrpc"
	"go.opencensus.io/stats/view"
	"go.opencensus.io/tag"
	"golang.org/x/net/http2"
	"golang.org/x/net/http2/h2c"
	"google.golang.org/grpc"
	"google.golang.org/grpc/health"
	healthpb "google.golang.org/grpc/health/grpc_health_v1"
	"google.golang.org/grpc/reflection"
)

var (
	KeyGRPCClient, _ = tag.NewKey("client")
)

func GRPCServerViewsPerClient() []*view.View {
	res := []*view.View{}
	for _, v := range ocgrpc.DefaultServerViews {
		customView := *v
		customView.Name = fmt.Sprintf("%s_per_client", customView.Name)
		customView.TagKeys = append(customView.TagKeys, KeyGRPCClient)
		res = append(res, &customView)
	}
	return res
}

func isGrpcRequest(r *http.Request) bool {
	return r.ProtoMajor == 2 && strings.HasPrefix(r.Header.Get("Content-Type"), "application/grpc")
}

func main() {
	flag.Parse()
	if err := view.Register(GRPCServerViewsPerClient()...); err != nil {
		glog.Fatal(err)
	}

	grpcServer := grpc.NewServer(grpc.StatsHandler(
		&ocgrpc.ServerHandler{},
	))
	healthServer := health.NewServer()
	healthpb.RegisterHealthServer(
		grpcServer,
		healthServer,
	)

	httpMux := http.NewServeMux()
	pe, _ := prometheus.NewExporter(prometheus.Options{
		Namespace: "",
	})
	httpMux.Handle("/metrics", pe)

	muxHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		clientIDValue := "foo@bar.com"
		ctx, err := tag.New(r.Context(),
			tag.Insert(KeyGRPCClient, clientIDValue),
		)
		if err != nil {
			glog.Errorf("Failed adding tag to context: %s", err)
			http.Error(w, err.Error(), 500)
			return
		}
		r = r.WithContext(ctx)
		if isGrpcRequest(r) {
			grpcServer.ServeHTTP(w, r)
			return
		}
		httpMux.ServeHTTP(w, r)
	})

	reflection.Register(grpcServer)
	listenAddr := "localhost:8000"
	glog.Infof("Listening on %s", listenAddr)
	log.Fatal(http.ListenAndServe(listenAddr, h2c.NewHandler(muxHandler, &http2.Server{})))
}

Then I send a grpc.health.v1.Health/Check() request and check the /metrics output.

What did you expect to see?

I expected the value for the KeyGRPCClient OC tag to show up in the ocgrpc view, i.e. get populated in the prom exporter hander as client="foo@bar.com" label

What did you see instead?

The value from the Go/OC context is not propagated to the ocgrpc context. The exported metrics have the label client=""

Additional context

I believe the problem is at

opencensus-go/plugin/ocgrpc/server_stats_handler.go

Line 43 in e736602

ctx = tag.NewContext(ctx, propagated)

, where the oc tag map in the context is overwritten with tag.NewContext() to populate it with the tags from grpc metadata.

Currently there's a NewContext() method in the tag module to overwrite the tags map and New() which is used with mutators to add/remove single value.

At a first glance the solution might be to add a new mutator to merge the provided map (similar to the one in New()) into the current context and use the mutator instead of tag.NewContext().

This looks somewhat related to #746 but this one seems to have an easy solution that doesn't introduce the mutability concerns.