stdoutmetric: Add WithWriter and WithPrettyPrint options (#4507)

* add WithWriter and WithPrettyOptions to stdoutmetric

* add changelog

* make WithWriter a shortcut to WithEncoder

* Update exporters/stdout/stdoutmetric/config.go

---------

Co-authored-by: Robert Pająk <pellared@hotmail.com>

CHANGELOG.md

@@ -8,9 +8,14 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ## [Unreleased]
 
+### Changed
+
+- `go.opentelemetry.io/otel/exporters/stdout/stdoutmetric` does not prettifies the output by default anymore. (#4507)
+
 ### Added
 
 - Add the "Roll the dice" getting started application example in `go.opentelemetry.io/otel/example/dice`. (#4539)
+- The `WithWriter` and `WithPrettyPrint` options to `go.opentelemetry.io/otel/exporters/stdout/stdoutmetric` to set a custom `io.Writer`, and allow displaying the output in human-readable JSON (#4507).
 
 ## [1.19.0-rc.1/0.42.0-rc.1] 2023-09-14
 

exporters/stdout/stdoutmetric/config.go

@@ -15,13 +15,15 @@ package stdoutmetric // import "go.opentelemetry.io/otel/exporters/stdout/stdout
 
 import (
 	"encoding/json"
+	"io"
 	"os"
 
 	"go.opentelemetry.io/otel/sdk/metric"
 )
 
 // config contains options for the exporter.
 type config struct {
+	prettyPrint         bool
 	encoder             *encoderHolder
 	temporalitySelector metric.TemporalitySelector
 	aggregationSelector metric.AggregationSelector
@@ -37,10 +39,15 @@ func newConfig(options ...Option) config {
 
 	if cfg.encoder == nil {
 		enc := json.NewEncoder(os.Stdout)
-		enc.SetIndent("", "\t")
 		cfg.encoder = &encoderHolder{encoder: enc}
 	}
 
+	if cfg.prettyPrint {
+		if e, ok := cfg.encoder.encoder.(*json.Encoder); ok {
+			e.SetIndent("", "\t")
+		}
+	}
+
 	if cfg.temporalitySelector == nil {
 		cfg.temporalitySelector = metric.DefaultTemporalitySelector
 	}
@@ -74,6 +81,22 @@ func WithEncoder(encoder Encoder) Option {
 	})
 }
 
+// WithWriter sets the export stream destination.
+// Using this option overrides any previously set encoder.
+func WithWriter(w io.Writer) Option {
+	return WithEncoder(json.NewEncoder(w))
+}
+
+// WithPrettyPrint prettifies the emitted output.
+// This option only works if the encoder is a *json.Encoder, as is the case
+// when using `WithWriter`.
+func WithPrettyPrint() Option {
+	return optionFunc(func(c config) config {
+		c.prettyPrint = true
+		return c
+	})
+}
+
 // WithTemporalitySelector sets the TemporalitySelector the exporter will use
 // to determine the Temporality of an instrument based on its kind. If this
 // option is not used, the exporter will use the DefaultTemporalitySelector

exporters/stdout/stdoutmetric/exporter_test.go

@@ -15,6 +15,7 @@
 package stdoutmetric_test // import "go.opentelemetry.io/otel/exporters/stdout/stdoutmetric"
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"io"
@@ -103,6 +104,43 @@ func deltaSelector(metric.InstrumentKind) metricdata.Temporality {
 	return metricdata.DeltaTemporality
 }
 
+func TestExportWithOptions(t *testing.T) {
+	var (
+		data = new(metricdata.ResourceMetrics)
+		ctx  = context.Background()
+	)
+
+	for _, tt := range []struct {
+		name string
+		opts []stdoutmetric.Option
+
+		expectedData string
+	}{
+		{
+			name:         "with no options",
+			expectedData: "{\"Resource\":null,\"ScopeMetrics\":null}\n",
+		},
+		{
+			name: "with pretty print",
+			opts: []stdoutmetric.Option{
+				stdoutmetric.WithPrettyPrint(),
+			},
+			expectedData: "{\n\t\"Resource\": null,\n\t\"ScopeMetrics\": null\n}\n",
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			var b bytes.Buffer
+			opts := append(tt.opts, stdoutmetric.WithWriter(&b))
+
+			exp, err := stdoutmetric.New(opts...)
+			require.NoError(t, err)
+			require.NoError(t, exp.Export(ctx, data))
+
+			assert.Equal(t, tt.expectedData, b.String())
+		})
+	}
+}
+
 func TestTemporalitySelector(t *testing.T) {
 	exp, err := stdoutmetric.New(
 		testEncoderOption(),