Don't publish testing -all jar. (#3493)

buildSrc/src/main/kotlin/io.opentelemetry.instrumentation.javaagent-testing.gradle.kts

@@ -40,7 +40,7 @@ val testInstrumentation by configurations.creating {
   isCanBeResolved = true
 }
 
-tasks.named<ShadowJar>("shadowJar").configure {
+tasks.shadowJar {
   configurations = listOf(project.configurations.runtimeClasspath.get(), testInstrumentation)
 
   archiveFileName.set("agent-testing.jar")
@@ -108,6 +108,19 @@ afterEvaluate {
   }
 }
 
+// shadowJar is only used for creating a jar for testing, but the shadow plugin automatically adds
+// it to a project's published Java component. Skip it if publishing is configured for this
+// project.
+plugins.withId("maven-publish") {
+  configure<PublishingExtension> {
+    (components["java"] as AdhocComponentWithVariants).run {
+      withVariantsFromConfiguration(configurations["shadowRuntimeElements"]) {
+        skip()
+      }
+    }
+  }
+}
+
 configurations.configureEach {
   if (name.toLowerCase().endsWith("testruntimeclasspath")) {
     // Added by agent, don't let Gradle bring it in when running tests.