Exercise: method and traits: change output (#2383)

In this exercise, the original StderrLogger output to stderr, which user
cannot see the original log but "No output" in frontend". Change
`eprintln!` to `println!` to make it see-able in the frontend. Also, due
to the change, rename the logger struct.

Close: #2382

Signed-off-by: Alx-Lai <alexabc722@gmail.com>

src/methods-and-traits/exercise.md

@@ -9,7 +9,7 @@ method. Code which might log its progress can then take an `&impl Logger`. In
 testing, this might put messages in the test logfile, while in a production
 build it would send messages to a log server.
 
-However, the `StderrLogger` given below logs all messages, regardless of
+However, the `StdoutLogger` given below logs all messages, regardless of
 verbosity. Your task is to write a `VerbosityFilter` type that will ignore
 messages above a maximum verbosity.
 

src/methods-and-traits/exercise.rs

@@ -19,19 +19,19 @@ pub trait Logger {
     fn log(&self, verbosity: u8, message: &str);
 }
 
-struct StderrLogger;
+struct StdoutLogger;
 
-impl Logger for StderrLogger {
+impl Logger for StdoutLogger {
     fn log(&self, verbosity: u8, message: &str) {
-        eprintln!("verbosity={verbosity}: {message}");
+        println!("verbosity={verbosity}: {message}");
     }
 }
 // ANCHOR_END: setup
 
 /// Only log messages up to the given verbosity level.
 struct VerbosityFilter {
     max_verbosity: u8,
-    inner: StderrLogger,
+    inner: StdoutLogger,
 }
 
 impl Logger for VerbosityFilter {
@@ -44,7 +44,7 @@ impl Logger for VerbosityFilter {
 
 // ANCHOR: main
 fn main() {
-    let logger = VerbosityFilter { max_verbosity: 3, inner: StderrLogger };
+    let logger = VerbosityFilter { max_verbosity: 3, inner: StdoutLogger };
     logger.log(5, "FYI");
     logger.log(2, "Uhoh");
 }