add examples of overriding formatters

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

examples/README.md

@@ -18,6 +18,10 @@ This directory contains a collection of examples that demonstrate the use of the
     which provides a subscriber implementation that logs traces to the console.
   + `fmt-stderr`: Demonstrates overriding the output stream used by the `fmt`
     subscriber.
+  + `fmt-custom-field`: Demonstrates overriding how the `fmt` subscriber formats
+    fields on spans and events.
+  + `fmt-custom-event`: Demonstrates overriding how the `fmt` subscriber formats
+    events.
   + `subscriber-filter`: Demonstrates the `tracing-subscriber::filter` module,
     which provides a layer which adds configurable filtering to a subscriber
     implementation.

examples/examples/fmt-custom-event.rs

@@ -0,0 +1,29 @@
+#![deny(rust_2018_idioms)]
+use tracing_subscriber;
+#[path = "fmt/yak_shave.rs"]
+mod yak_shave;
+
+fn main() {
+    use tracing_subscriber::fmt;
+
+    // Configure a custom event formatter
+    let format = fmt::format()
+        .compact() // use an abbreviated format for logging spans
+        .with_level(false) // don't include levels in formatted output
+        .with_target(false); // don't include targets
+
+    // Create a `fmt` subscriber that uses our custom event format, and set it
+    // as the default.
+    fmt().event_format(format).init();
+
+    // Shave some yaks!
+    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."
+    );
+}

examples/examples/fmt-custom-field.rs

@@ -0,0 +1,37 @@
+//! This example demonstrates overriding the way `tracing-subscriber`'s
+//! `FmtSubscriber` formats fields on spans and events, using a closure.
+#![deny(rust_2018_idioms)]
+use tracing::Level;
+use tracing_subscriber;
+
+#[path = "fmt/yak_shave.rs"]
+mod yak_shave;
+
+fn main() {
+    use tracing_subscriber::{fmt::format, prelude::*};
+
+    // Format fields using the provided closure.
+    let format = format::debug_fn(|writer, _field, value| {
+        // We'll format _just_ the value of each field, ignoring the field name.
+        write!(writer, "{:?}", value)
+    })
+    // Separate each field with a comma.
+    // This method is provided by an extension trait in the
+    // `tracing-subscriber` prelude.
+    .delimited(", ");
+
+    // Create a `fmt` subscriber that uses our custom event format, and set it
+    // as the default.
+    tracing_subscriber::fmt().fmt_fields(format).init();
+
+    // Shave some yaks!
+    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."
+    );
+}