Add schema URL support to Tracer (#1889)

This adds support for schema URL to the Tracer according to the specification:
https://github.com/open-telemetry/opentelemetry-specification/pull/1666
(Link to replaced by the link to the spec after that PR is merged)

For the future: once the proto is updated we will need to populate the
schema_url field in the messages.

sdk/trace/provider_test.go

@@ -20,6 +20,8 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+
+	"go.opentelemetry.io/otel/trace"
 )
 
 type basicSpanProcesor struct {
@@ -82,3 +84,13 @@ func TestFailedProcessorShutdownInUnregister(t *testing.T) {
 	err := stp.Shutdown(context.Background())
 	assert.NoError(t, err)
 }
+
+func TestSchemaURL(t *testing.T) {
+	stp := NewTracerProvider()
+	schemaURL := "https://opentelemetry.io/schemas/1.2.0"
+	tracerIface := stp.Tracer("tracername", trace.WithSchemaURL(schemaURL))
+
+	// Verify that the SchemaURL of the constructed Tracer is correctly populated.
+	tracerStruct := tracerIface.(*tracer)
+	assert.EqualValues(t, schemaURL, tracerStruct.instrumentationLibrary.SchemaURL)
+}