Use slf4j instead of log4j in javaagent example (#518)

Author: jack-berg

javaagent/src/main/java/io/opentelemetry/example/javagent/Controller.java

@@ -9,16 +9,16 @@
 import io.opentelemetry.api.trace.Tracer;
 import io.opentelemetry.context.Scope;
 import java.util.Random;
-import org.apache.logging.log4j.LogManager;
-import org.apache.logging.log4j.Logger;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.web.bind.annotation.GetMapping;
 import org.springframework.web.bind.annotation.RestController;
 
 @RestController
 public class Controller {
 
-  private static final Logger LOGGER = LogManager.getLogger(Controller.class);
+  private static final Logger LOGGER = LoggerFactory.getLogger(Controller.class);
   private final AttributeKey<String> ATTR_METHOD = AttributeKey.stringKey("method");
 
   private final Random random = new Random();