add RegisterSimpleSpanProcessor() modeled on stackdriver code (#213)

* add RegisterSimpleSpanProcessor() modeled on stackdriver code

* update examples

example/http/client/client.go

@@ -43,10 +43,7 @@ func initTracer() {
 	if err != nil {
 		log.Fatal(err)
 	}
-
-	// Wrap stdout exporter with SimpleSpanProcessor and register the processor.
-	ssp := sdktrace.NewSimpleSpanProcessor(exporter)
-	sdktrace.RegisterSpanProcessor(ssp)
+	exporter.RegisterSimpleSpanProcessor()
 
 	// For the demonstration, use sdktrace.AlwaysSample sampler to sample all traces.
 	// In a production application, use sdktrace.ProbabilitySampler with a desired probability.

example/http/server/server.go

@@ -35,10 +35,7 @@ func initTracer() {
 	if err != nil {
 		log.Fatal(err)
 	}
-
-	// Wrap stdout exporter with SimpleSpanProcessor and register the processor.
-	ssp := sdktrace.NewSimpleSpanProcessor(exporter)
-	sdktrace.RegisterSpanProcessor(ssp)
+	exporter.RegisterSimpleSpanProcessor()
 
 	// For the demonstration, use sdktrace.AlwaysSample sampler to sample all traces.
 	// In a production application, use sdktrace.ProbabilitySampler with a desired probability.

exporter/trace/jaeger/example/main.go

@@ -39,10 +39,7 @@ func main() {
 	if err != nil {
 		log.Fatal(err)
 	}
-
-	// Wrap exporter with SimpleSpanProcessor and register the processor.
-	ssp := trace.NewSimpleSpanProcessor(exporter)
-	trace.RegisterSpanProcessor(ssp)
+	exporter.RegisterSimpleSpanProcessor()
 
 	// For demoing purposes, always sample. In a production application, you should
 	// configure this to a trace.ProbabilitySampler set at the desired

exporter/trace/jaeger/jaeger.go

@@ -17,13 +17,15 @@ package jaeger
 import (
 	"context"
 	"log"
+	"sync"
 
 	"google.golang.org/api/support/bundler"
 	"google.golang.org/grpc/codes"
 
 	"go.opentelemetry.io/api/core"
 	gen "go.opentelemetry.io/exporter/trace/jaeger/internal/gen-go/jaeger"
 	"go.opentelemetry.io/sdk/export"
+	"go.opentelemetry.io/sdk/trace"
 )
 
 const defaultServiceName = "OpenTelemetry"
@@ -138,11 +140,20 @@ type Tag struct {
 
 // Exporter is an implementation of trace.Exporter that uploads spans to Jaeger.
 type Exporter struct {
+	once     sync.Once
 	process  *gen.Process
 	bundler  *bundler.Bundler
 	uploader batchUploader
 }
 
+// RegisterSimpleSpanProcessor registers e as SimpleSpanProcessor.
+func (e *Exporter) RegisterSimpleSpanProcessor() {
+	e.once.Do(func() {
+		ssp := trace.NewSimpleSpanProcessor(e)
+		trace.RegisterSpanProcessor(ssp)
+	})
+}
+
 var _ export.SpanSyncer = (*Exporter)(nil)
 
 // ExportSpan exports a SpanData to Jaeger.

exporter/trace/stdout/stdout.go

@@ -19,8 +19,10 @@ import (
 	"encoding/json"
 	"io"
 	"os"
+	"sync"
 
 	"go.opentelemetry.io/sdk/export"
+	"go.opentelemetry.io/sdk/trace"
 )
 
 // Options are the options to be used when initializing a stdout export.
@@ -32,6 +34,7 @@ type Options struct {
 
 // Exporter is an implementation of trace.Exporter that writes spans to stdout.
 type Exporter struct {
+	once         sync.Once
 	pretty       bool
 	outputWriter io.Writer
 }
@@ -43,6 +46,14 @@ func NewExporter(o Options) (*Exporter, error) {
 	}, nil
 }
 
+// RegisterSimpleSpanProcessor registers e as SimpleSpanProcessor.
+func (e *Exporter) RegisterSimpleSpanProcessor() {
+	e.once.Do(func() {
+		ssp := trace.NewSimpleSpanProcessor(e)
+		trace.RegisterSpanProcessor(ssp)
+	})
+}
+
 // ExportSpan writes a SpanData in json format to stdout.
 func (e *Exporter) ExportSpan(ctx context.Context, data *export.SpanData) {
 	var jsonSpan []byte