Add additional fields example

examples/README.md

@@ -37,6 +37,8 @@ This directory contains a collection of examples that demonstrate the use of the
     both the terminal and the system journal.
   + `toggle-subscribers` : Demonstrates how subscribers can be wrapped with an `Option` allowing
     them to be dynamically toggled.
+  + `fmt-additional-fields`: Demonstrates how to create a subscriber that enriches
+    the `fmt` collector with additional information.
 - **tracing-futures**:
   + `spawny-thing`: Demonstrates the use of the `#[instrument]` attribute macro
     asynchronous functions.

examples/examples/fmt-additional-fields.rs

@@ -0,0 +1,73 @@
+#![deny(rust_2018_idioms)]
+
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+use tracing::{span, Collect};
+use tracing_subscriber::{
+    fmt::format::AdditionalFmtSpanFields,
+    registry::LookupSpan,
+    subscribe::{CollectExt, Context},
+    util::SubscriberInitExt,
+    Subscribe,
+};
+#[path = "fmt/yak_shave.rs"]
+mod yak_shave;
+
+struct CountingSubscribe(AtomicUsize);
+
+impl<C: Collect + for<'lookup> LookupSpan<'lookup>> Subscribe<C> for CountingSubscribe {
+    fn on_new_span(&self, _attrs: &span::Attributes<'_>, id: &span::Id, ctx: Context<'_, C>) {
+        // Find the span.
+        let span = ctx
+            .span(id)
+            .expect("The span should exist in the registry.");
+
+        // Get its extensions.
+        let mut extensions = span.extensions_mut();
+
+        // Find the additional fields in the extensions or create new ones. It's
+        // important to always look for a previous value as another layer may
+        // have already added some fields.
+        let mut additional_fields = extensions
+            .remove::<AdditionalFmtSpanFields>()
+            .unwrap_or_default();
+
+        // Add something to the fields.
+        let ordinal = self.0.fetch_add(1, Ordering::Relaxed);
+        additional_fields.insert("ordinal".to_owned(), ordinal.to_string());
+
+        // And don't forget to then put the additional fields into the extensions!
+        extensions.insert(additional_fields);
+    }
+}
+
+fn main() {
+    tracing_subscriber::fmt()
+        // Use json output...
+        .json()
+        // Enable additional span fields.
+        .with_additional_span_fields(true)
+        // Disable things not needed for the example to make the output more readable.
+        .with_span_list(false)
+        .without_time()
+        .with_target(false)
+        // Enable all levels.
+        .with_max_level(tracing::Level::TRACE)
+        // Create the collector...
+        .finish()
+        // and add our enriching subscriber as another layer.
+        // Try removing this and see what changes in the output!
+        .with(CountingSubscribe(AtomicUsize::new(0)))
+        // Set this to be the default, global collector for this application.
+        .init();
+
+    let number_of_yaks = 3;
+    // this creates a new event, outside of any spans.
+    tracing::info!(number_of_yaks, "preparing to shave yaks");
+
+    let number_shaved = yak_shave::shave_all(number_of_yaks);
+    tracing::info!(
+        all_yaks_shaved = number_shaved == number_of_yaks,
+        "yak shaving completed."
+    );
+}