Add example for SetTextMapPropagator (#6579)

Example demonstrating how to use `SetTextMapPropagator` with a composite
propagator

Related Issue: Closes #6571

Co-authored-by: Damien Mathieu <42@dmathieu.com>

propagation/example_test.go

@@ -0,0 +1,20 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package propagation_test
+
+import (
+	"go.opentelemetry.io/otel"
+	"go.opentelemetry.io/otel/propagation"
+)
+
+func ExampleSetTextMapPropagator() {
+	// Create a new composite text map propagator.
+	propagator := propagation.NewCompositeTextMapPropagator(
+		propagation.TraceContext{},
+		propagation.Baggage{},
+	)
+
+	// Set it as the global text map propagator.
+	otel.SetTextMapPropagator(propagator)
+}