Add support for tracing via OpenCencus

This adds a few flags for configuring the tracer.
Includes support for jaeger tracing (built into OC).

vendor/go.opencensus.io/plugin/ocgrpc/client_stats_handler.go

@@ -0,0 +1,49 @@
+// Copyright 2017, 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 ocgrpc
+
+import (
+	"time"
+
+	"go.opencensus.io/tag"
+	"golang.org/x/net/context"
+	"google.golang.org/grpc/grpclog"
+	"google.golang.org/grpc/stats"
+)
+
+// statsTagRPC gets the tag.Map populated by the application code, serializes
+// its tags into the GRPC metadata in order to be sent to the server.
+func (h *ClientHandler) statsTagRPC(ctx context.Context, info *stats.RPCTagInfo) context.Context {
+	startTime := time.Now()
+	if info == nil {
+		if grpclog.V(2) {
+			grpclog.Infof("clientHandler.TagRPC called with nil info.", info.FullMethodName)
+		}
+		return ctx
+	}
+
+	d := &rpcData{
+		startTime: startTime,
+		method:    info.FullMethodName,
+	}
+	ts := tag.FromContext(ctx)
+	if ts != nil {
+		encoded := tag.Encode(ts)
+		ctx = stats.SetTags(ctx, encoded)
+	}
+
+	return context.WithValue(ctx, rpcDataKey, d)
+}