add definition for standard output span exporter (#3740)

CHANGELOG.md

@@ -11,6 +11,9 @@ release.
 
 ### Traces
 
+- Add definition for standard output span exporter.
+  ([#3740](https://github.com/open-telemetry/opentelemetry-specification/pull/3740))
+
 ### Metrics
 
 ### Logs

specification/trace/sdk_exporters/stdout.md

@@ -0,0 +1,22 @@
+<!--- Hugo front matter used to generate the website version of this page:
+linkTitle: Stdout
+--->
+
+# OpenTelemetry Span Exporter - Standard output
+
+**Status**: [Stable](../../document-status.md)
+
+"Standard output" Span Exporter is a [Span
+Exporter](../sdk.md#span-exporter) which outputs the spans to
+stdout/console.
+
+[OpenTelemetry SDK](../../overview.md#sdk) authors MAY choose the best idiomatic
+name for their language. For example, ConsoleExporter, StdoutExporter,
+StreamExporter, LoggingExporter etc.
+
+If a language provides a mechanism to automatically configure a
+[Span processor](../sdk.md#span-processor) to pair with the associated
+exporter (e.g., using the [`OTEL_TRACES_EXPORTER` environment
+variable](../../configuration/sdk-environment-variables.md#exporter-selection)), by
+default the standard output exporter SHOULD be paired with a [simple
+processor](../sdk.md#simple-processor).