fix(agent): Avoid using stdout to report error

Using stdout instead of stderr breaks CLI tools used in shell sub-commands if anything goes wrong.

dd-java-agent/src/main/java/datadog/trace/bootstrap/AgentBootstrap.java

@@ -116,7 +116,7 @@ static boolean exceptionCauseChainContains(Throwable ex, String exClassName) {
 
   private static boolean alreadyInitialized() {
     if (initialized) {
-      System.out.println(
+      System.err.println(
           "Warning: dd-java-agent is being initialized more than once. Please check that you are defining -javaagent:dd-java-agent.jar only once.");
       return true;
     }
@@ -125,7 +125,7 @@ private static boolean alreadyInitialized() {
   }
 
   private static boolean lessThanJava8() {
-    return lessThanJava8(System.getProperty("java.version"), System.out);
+    return lessThanJava8(System.getProperty("java.version"), System.err);
   }
 
   // Reachable for testing
@@ -234,7 +234,7 @@ && getAgentFilesFromVMArguments().size() > 1) {
         agentFiles.append(agentFile.getAbsolutePath());
         agentFiles.append('"');
       }
-      System.out.println(
+      System.err.println(
           "Info: multiple JVM agents detected, found "
               + agentFiles
               + ". Loading multiple APM/Tracing agent is not a recommended or supported configuration."
@@ -267,14 +267,14 @@ private static synchronized URL installAgentJar(final Instrumentation inst)
       }
     }
 
-    System.out.println("Could not get bootstrap jar from code source, using -javaagent arg");
+    System.err.println("Could not get bootstrap jar from code source, using -javaagent arg");
     File javaagentFile = getAgentFileFromJavaagentArg(getAgentFilesFromVMArguments());
     if (javaagentFile != null) {
       URL ddJavaAgentJarURL = javaagentFile.toURI().toURL();
       return appendAgentToBootstrapClassLoaderSearch(inst, ddJavaAgentJarURL, javaagentFile);
     }
 
-    System.out.println(
+    System.err.println(
         "Could not get agent jar from -javaagent arg, using ClassLoader#getResource");
     javaagentFile = getAgentFileUsingClassLoaderLookup();
     if (!javaagentFile.isDirectory()) {
@@ -295,10 +295,10 @@ private static URL appendAgentToBootstrapClassLoaderSearch(
 
   private static File getAgentFileFromJavaagentArg(List<File> agentFiles) {
     if (agentFiles.isEmpty()) {
-      System.out.println("Could not get bootstrap jar from -javaagent arg: no argument specified");
+      System.err.println("Could not get bootstrap jar from -javaagent arg: no argument specified");
       return null;
     } else if (agentFiles.size() > 1) {
-      System.out.println(
+      System.err.println(
           "Could not get bootstrap jar from -javaagent arg: multiple javaagents specified");
       return null;
     } else {
@@ -324,7 +324,7 @@ private static List<File> getAgentFilesFromVMArguments() {
           if (agentFile.exists() && agentFile.isFile()) {
             agentFiles.add(agentFile);
           } else {
-            System.out.println(
+            System.err.println(
                 "Could not get bootstrap jar from -javaagent arg: unable to find javaagent file: "
                     + agentFile);
           }
@@ -395,13 +395,13 @@ private static List<String> getVMArgumentsThroughReflection() {
 
     // Fallback to default
     try {
-      System.out.println(
+      System.err.println(
           "WARNING: Unable to get VM args through reflection. A custom java.util.logging.LogManager may not work correctly");
       return ManagementFactory.getRuntimeMXBean().getInputArguments();
     } catch (final Throwable t) {
       // Throws InvocationTargetException on modularized applications
       // with non-opened java.management module
-      System.out.println("WARNING: Unable to get VM args using managed beans");
+      System.err.println("WARNING: Unable to get VM args using managed beans");
     }
     return Collections.emptyList();
   }