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/exporter/jaeger/agent.go

@@ -0,0 +1,89 @@
+// 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 jaeger
+
+import (
+	"fmt"
+	"io"
+	"net"
+
+	"git.apache.org/thrift.git/lib/go/thrift"
+	gen "go.opencensus.io/exporter/jaeger/internal/gen-go/jaeger"
+)
+
+// udpPacketMaxLength is the max size of UDP packet we want to send, synced with jaeger-agent
+const udpPacketMaxLength = 65000
+
+// agentClientUDP is a UDP client to Jaeger agent that implements gen.Agent interface.
+type agentClientUDP struct {
+	gen.Agent
+	io.Closer
+
+	connUDP       *net.UDPConn
+	client        *gen.AgentClient
+	maxPacketSize int                   // max size of datagram in bytes
+	thriftBuffer  *thrift.TMemoryBuffer // buffer used to calculate byte size of a span
+}
+
+// newAgentClientUDP creates a client that sends spans to Jaeger Agent over UDP.
+func newAgentClientUDP(hostPort string, maxPacketSize int) (*agentClientUDP, error) {
+	if maxPacketSize == 0 {
+		maxPacketSize = udpPacketMaxLength
+	}
+
+	thriftBuffer := thrift.NewTMemoryBufferLen(maxPacketSize)
+	protocolFactory := thrift.NewTCompactProtocolFactory()
+	client := gen.NewAgentClientFactory(thriftBuffer, protocolFactory)
+
+	destAddr, err := net.ResolveUDPAddr("udp", hostPort)
+	if err != nil {
+		return nil, err
+	}
+
+	connUDP, err := net.DialUDP(destAddr.Network(), nil, destAddr)
+	if err != nil {
+		return nil, err
+	}
+	if err := connUDP.SetWriteBuffer(maxPacketSize); err != nil {
+		return nil, err
+	}
+
+	clientUDP := &agentClientUDP{
+		connUDP:       connUDP,
+		client:        client,
+		maxPacketSize: maxPacketSize,
+		thriftBuffer:  thriftBuffer}
+	return clientUDP, nil
+}
+
+// EmitBatch implements EmitBatch() of Agent interface
+func (a *agentClientUDP) EmitBatch(batch *gen.Batch) error {
+	a.thriftBuffer.Reset()
+	a.client.SeqId = 0 // we have no need for distinct SeqIds for our one-way UDP messages
+	if err := a.client.EmitBatch(batch); err != nil {
+		return err
+	}
+	if a.thriftBuffer.Len() > a.maxPacketSize {
+		return fmt.Errorf("Data does not fit within one UDP packet; size %d, max %d, spans %d",
+			a.thriftBuffer.Len(), a.maxPacketSize, len(batch.Spans))
+	}
+	_, err := a.connUDP.Write(a.thriftBuffer.Bytes())
+	return err
+}
+
+// Close implements Close() of io.Closer and closes the underlying UDP connection.
+func (a *agentClientUDP) Close() error {
+	return a.connUDP.Close()
+}