Add example for Zipkin exporter (#602)

Fixes #531.

exporter/zipkin/example/main.go

@@ -0,0 +1,75 @@
+// 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 main
+
+import (
+	"context"
+	"log"
+	"time"
+
+	openzipkin "github.com/openzipkin/zipkin-go"
+	"github.com/openzipkin/zipkin-go/reporter/http"
+	"go.opencensus.io/exporter/zipkin"
+	"go.opencensus.io/trace"
+)
+
+func main() {
+	// The localEndpoint stores the name and address of the local service
+	localEndpoint, err := openzipkin.NewEndpoint("example-server", "192.168.1.5:5454")
+	if err != nil {
+		log.Println(err)
+	}
+
+	// The Zipkin reporter takes collected spans from the app and reports them to the backend
+	// http://localhost:9411/api/v2/spans is the default for the Zipkin Span v2
+	reporter := http.NewReporter("http://localhost:9411/api/v2/spans")
+	defer reporter.Close()
+
+	// The OpenCensus exporter wraps the Zipkin reporter
+	exporter := zipkin.NewExporter(reporter, localEndpoint)
+	trace.RegisterExporter(exporter)
+
+	// For example purposes, sample every trace.
+	trace.SetDefaultSampler(trace.AlwaysSample())
+
+	ctx := context.Background()
+	foo(ctx)
+}
+
+func foo(ctx context.Context) {
+	// Name the current span "/foo"
+	ctx, span := trace.StartSpan(ctx, "/foo")
+	defer span.End()
+
+	// Foo calls bar and baz
+	bar(ctx)
+	baz(ctx)
+}
+
+func bar(ctx context.Context) {
+	ctx, span := trace.StartSpan(ctx, "/bar")
+	defer span.End()
+
+	// Do bar
+	time.Sleep(2 * time.Millisecond)
+}
+
+func baz(ctx context.Context) {
+	ctx, span := trace.StartSpan(ctx, "/baz")
+	defer span.End()
+
+	// Do baz
+	time.Sleep(4 * time.Millisecond)
+}