feat(example): show milliseconds since logger init

examples/millis_since_init.rs

@@ -0,0 +1,40 @@
+/*!
+Including the milliseconds elapsed since the logger was initialized in each
+record, similar to `Log4J`'s relative time pattern.
+
+Before running this example, try setting the `MY_LOG_LEVEL` environment variable to `info`:
+
+```no_run,shell
+$ export MY_LOG_LEVEL='info'
+```
+*/
+
+use std::io::Write;
+use std::time::{Duration, Instant};
+
+use env_logger::{Builder, Env};
+
+fn init_logger() {
+    let env = Env::default().filter("MY_LOG_LEVEL");
+
+    let start = Instant::now();
+    Builder::from_env(env)
+        .format(move |buf, record| {
+            let elapsed = start.elapsed().as_millis();
+            writeln!(
+                buf,
+                "[{elapsed:>6} ms] {}: {}",
+                record.level(),
+                record.args()
+            )
+        })
+        .init();
+}
+
+fn main() {
+    init_logger();
+
+    log::info!("a log from `MyLogger`");
+    std::thread::sleep(Duration::from_millis(250));
+    log::warn!("another log, a moment later");
+}