[trace] add option.Writer for stdout output (#404)

* add option.File for stdout output

* Unify the naming of `Writer` field (io.Writer) in Options

api/global/internal/meter_test.go

@@ -222,7 +222,7 @@ func TestDefaultSDK(t *testing.T) {
 
 		return pusher
 	}(stdout.Options{
-		File:           out,
+		Writer:         out,
 		DoNotPrintTime: true,
 	})
 

exporter/metric/stdout/stdout.go

@@ -35,8 +35,8 @@ var _ export.Exporter = &Exporter{}
 
 // Options are the options to be used when initializing a stdout export.
 type Options struct {
-	// File is the destination.  If not set, os.Stdout is used.
+	// Writer is the destination.  If not set, os.Stdout is used.
-	File io.Writer
+	Writer io.Writer
 
 	// PrettyPrint will pretty the json representation of the span,
 	// making it print "pretty". Default is false.
@@ -81,8 +81,8 @@ type expoQuantile struct {
 }
 
 func New(options Options) (*Exporter, error) {
-	if options.File == nil {
+	if options.Writer == nil {
-		options.File = os.Stdout
+		options.Writer = os.Stdout
 	}
 	if options.Quantiles == nil {
 		options.Quantiles = []float64{0.5, 0.9, 0.99}
@@ -218,7 +218,7 @@ func (e *Exporter) Export(_ context.Context, checkpointSet export.CheckpointSet)
 	}
 
 	if err == nil {
-		fmt.Fprintln(e.options.File, string(data))
+		fmt.Fprintln(e.options.Writer, string(data))
 	} else {
 		return err
 	}

exporter/metric/stdout/stdout_test.go

@@ -34,7 +34,7 @@ type testFixture struct {
 
 func newFixture(t *testing.T, options stdout.Options) testFixture {
 	buf := &bytes.Buffer{}
-	options.File = buf
+	options.Writer = buf
 	options.DoNotPrintTime = true
 	exp, err := stdout.New(options)
 	if err != nil {
@@ -70,7 +70,7 @@ func TestStdoutInvalidQuantile(t *testing.T) {
 func TestStdoutTimestamp(t *testing.T) {
 	var buf bytes.Buffer
 	exporter, err := stdout.New(stdout.Options{
-		File:           &buf,
+		Writer:         &buf,
 		DoNotPrintTime: false,
 	})
 	if err != nil {

exporter/trace/stdout/stdout.go

@@ -25,6 +25,9 @@ import (
 
 // Options are the options to be used when initializing a stdout export.
 type Options struct {
+	// Writer is the destination.  If not set, os.Stdout is used.
+	Writer io.Writer
+
 	// PrettyPrint will pretty the json representation of the span,
 	// making it print "pretty". Default is false.
 	PrettyPrint bool
@@ -37,9 +40,12 @@ type Exporter struct {
 }
 
 func NewExporter(o Options) (*Exporter, error) {
+	if o.Writer == nil {
+		o.Writer = os.Stdout
+	}
 	return &Exporter{
 		pretty:       o.PrettyPrint,
-		outputWriter: os.Stdout,
+		outputWriter: o.Writer,
 	}, nil
 }
 

exporter/trace/stdout/stdout_test.go

@@ -30,15 +30,13 @@ import (
 )
 
 func TestExporter_ExportSpan(t *testing.T) {
-	ex, err := NewExporter(Options{})
+	// write to buffer for testing
+	var b bytes.Buffer
+	ex, err := NewExporter(Options{Writer: &b})
 	if err != nil {
 		t.Errorf("Error constructing stdout exporter %s", err)
 	}
 
-	// override output writer for testing
-	var b bytes.Buffer
-	ex.outputWriter = &b
-
 	// setup test span
 	now := time.Now()
 	traceID, _ := core.TraceIDFromHex("0102030405060708090a0b0c0d0e0f10")