functions/logging: add stdout/stderr + ignored logs severity note (#2422

)

~@vinod-at-google @labtopia IIRC the lack of log severity info is a known issue. (If it's not, let me know so I can hold off on merging this PR.)~

functions/snippets/src/main/java/com/example/functions/LogHelloWorld.java

@@ -32,6 +32,10 @@ public class LogHelloWorld implements HttpFunction {
   @Override
   public void service(HttpRequest request, HttpResponse response)
       throws IOException {
+    System.out.println("I am a log to stdout!");
+    System.err.println("I am a log to stderr!");
+
+    // Stackdriver does not detect severity from Cloud Functions log output
     LOGGER.info("I am an info log!");
     LOGGER.warning("I am a warning log!");
 

functions/snippets/src/test/java/com/example/functions/SnippetsTests.java

@@ -90,7 +90,7 @@ public void beforeTest() throws IOException {
 
   @After
   public void afterTest() {
-    System.setOut(null);
+    System.out.flush();
     logHandler.flush();
   }