add Empty field and record example

Author: seyyed-dev

examples/deferred.rs

@@ -1,3 +1,4 @@
+use tracing::field::Empty;
 use tracing::{debug, error, info, level_filters::LevelFilter, span, trace, warn, Level};
 use tracing_subscriber::{layer::SubscriberExt, registry::Registry, Layer};
 use tracing_tree::HierarchicalLayer;
@@ -28,6 +29,13 @@ fn main() {
 
     println!("-> This prints before the span open message");
 
+    let lazy = span!(Level::DEBUG, "lazy span", work_units = Empty);
+    lazy.record("work_units", &3);
+    lazy.in_scope(|| {
+        info!("doing some work");
+    });
+    drop(lazy);
+
     let _e2 = server_span.enter();
     info!("starting");
     std::thread::sleep(std::time::Duration::from_millis(1000));

examples/deferred.stdout

@@ -1,6 +1,11 @@
 -> This prints before the span open message
 1:main┐open: deferred::hierarchical-example version=0.1
-1:main└─┐open: deferred::server host="localhost", port=8080
+1:main└─┐open: deferred::lazy span 
+1:main  ├─ INFO deferred doing some work
+1:main ┌┘close(v): deferred::lazy span 
+1:main┌┘post_close: deferred::hierarchical-example version=0.1
+1:main└┐pre_open: deferred::hierarchical-example version=0.1
+1:main └┐open(v): deferred::server host="localhost", port=8080
 1:main  ├─ INFO deferred starting
 1:main  ├─ INFO deferred listening
 -> Deferring two levels of spans