ref(log): Extract Sentry tracing layer setup into a function

objectstore-log/src/subscriber.rs

@@ -13,33 +13,20 @@ include!(concat!(env!("OUT_DIR"), "/constants.gen.rs"));
 /// by `config.format`, defaulting to pretty when a terminal is attached.
 ///
 /// When built with the `sentry` feature, also attaches Sentry tracing layers if the Sentry client
-/// has been initialized. `ERROR`..`DEBUG` spans become Sentry spans and `ERROR`..`DEBUG` events
-/// become Sentry logs. `ERROR` and `WARN` events also become Sentry error events. Logs and error
-/// events include the attributes of their parent spans. `TRACE` spans and events are ignored.
+/// has been initialized:
+///
+/// - `ERROR`..`DEBUG` spans become Sentry spans.
+/// - `TRACE` spans are ignored.
+/// - `ERROR`..`DEBUG` events become Sentry logs.
+/// - `ERROR` and `WARN` events also become Sentry error events.
+/// - `TRACE` events are ignored.
+/// - Logs and error events include the attributes of their parent spans.
 pub fn init(config: &LoggingConfig) {
     #[cfg(feature = "sentry")]
     let sentry_layer = sentry::Hub::current()
         .client()
         .filter(|c| c.is_enabled())
-        .map(|_| {
-            use sentry::integrations::tracing::{ErrorLayer, LogLayer, SpanLayer};
-            use tracing_subscriber::filter::{FilterExt, LevelFilter, filter_fn};
-
-            SpanLayer::new()
-                .with_filter(LevelFilter::DEBUG)
-                .and_then(
-                    ErrorLayer::new()
-                        .enable_span_attributes()
-                        // Span attributes come only from spans that pass both the span layer's
-                        // and the event layer's filter, so the error layer accepts all spans.
-                        .with_filter(LevelFilter::WARN.or(filter_fn(|m| m.is_span()))),
-                )
-                .and_then(
-                    LogLayer::new()
-                        .enable_span_attributes()
-                        .with_filter(LevelFilter::DEBUG),
-                )
-        });
+        .map(|_| sentry_layers());
 
     let format = tracing_subscriber::fmt::layer()
         .with_writer(std::io::stderr)
@@ -82,6 +69,34 @@ pub fn init(config: &LoggingConfig) {
         .init();
 }
 
+/// Builds the Sentry tracing layers that [`init`] attaches, with the filtering described there.
+#[cfg(feature = "sentry")]
+fn sentry_layers<S>() -> impl tracing_subscriber::Layer<S>
+where
+    S: tracing::Subscriber + for<'a> tracing_subscriber::registry::LookupSpan<'a> + 'static,
+{
+    use sentry::integrations::tracing::{ErrorLayer, LogLayer, SpanLayer};
+    use tracing_subscriber::filter::{FilterExt, LevelFilter, filter_fn};
+
+    // Span attributes come only from spans that pass both the SpanLayer's and the ErrorLayer's
+    // filter, so the ErrorLayer accepts all spans. The LogLayer's filter already admits the
+    // same spans as the SpanLayer's.
+    let error_filter = LevelFilter::WARN.or(filter_fn(|metadata| metadata.is_span()));
+
+    SpanLayer::new()
+        .with_filter(LevelFilter::DEBUG)
+        .and_then(
+            ErrorLayer::new()
+                .enable_span_attributes()
+                .with_filter(error_filter),
+        )
+        .and_then(
+            LogLayer::new()
+                .enable_span_attributes()
+                .with_filter(LevelFilter::DEBUG),
+        )
+}
+
 /// Builds the default [`EnvFilter`] when `RUST_LOG` is not set.
 ///
 /// Uses `INFO` as the base level. Selected dependencies are raised to `DEBUG` where their debug