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.go

@@ -0,0 +1,56 @@
+// 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 ocgrpc
+
+import (
+	"go.opencensus.io/trace"
+	"golang.org/x/net/context"
+
+	"google.golang.org/grpc/stats"
+)
+
+// ClientHandler implements a gRPC stats.Handler for recording OpenCensus stats and
+// traces. Use with gRPC clients only.
+type ClientHandler struct {
+	// StartOptions allows configuring the StartOptions used to create new spans.
+	//
+	// StartOptions.SpanKind will always be set to trace.SpanKindClient
+	// for spans started by this handler.
+	StartOptions trace.StartOptions
+}
+
+// HandleConn exists to satisfy gRPC stats.Handler.
+func (c *ClientHandler) HandleConn(ctx context.Context, cs stats.ConnStats) {
+	// no-op
+}
+
+// TagConn exists to satisfy gRPC stats.Handler.
+func (c *ClientHandler) TagConn(ctx context.Context, cti *stats.ConnTagInfo) context.Context {
+	// no-op
+	return ctx
+}
+
+// HandleRPC implements per-RPC tracing and stats instrumentation.
+func (c *ClientHandler) HandleRPC(ctx context.Context, rs stats.RPCStats) {
+	traceHandleRPC(ctx, rs)
+	statsHandleRPC(ctx, rs)
+}
+
+// TagRPC implements per-RPC context management.
+func (c *ClientHandler) TagRPC(ctx context.Context, rti *stats.RPCTagInfo) context.Context {
+	ctx = c.traceTagRPC(ctx, rti)
+	ctx = c.statsTagRPC(ctx, rti)
+	return ctx
+}