don't add the test exporters when not needed (open-telemetry#14431)

Author: zeitlinger

testing-common/src/main/java/io/opentelemetry/instrumentation/testing/provider/TestExporterCustomizerProvider.java

@@ -28,6 +28,10 @@
 public class TestExporterCustomizerProvider implements DeclarativeConfigurationCustomizerProvider {
   @Override
   public void customize(DeclarativeConfigurationCustomizer customizer) {
+    if (TestSpanExporterComponentProvider.getSpanExporter() == null) {
+      // library test runner is not used, so we should not add the test exporters
+      return;
+    }
     customizer.addModelCustomizer(
         model -> {
           if (model.getTracerProvider() == null) {

testing-common/src/main/java/io/opentelemetry/instrumentation/testing/provider/TestSpanExporterComponentProvider.java

@@ -29,6 +29,10 @@ public SpanExporter create(DeclarativeConfigProperties config) {
     return Objects.requireNonNull(spanExporter, "spanExporter must not be null");
   }
 
+  public static SpanExporter getSpanExporter() {
+    return spanExporter;
+  }
+
   public static void setSpanExporter(SpanExporter spanExporter) {
     TestSpanExporterComponentProvider.spanExporter = spanExporter;
   }