feat(appender-tracing): stabilize span attribute propagation

opentelemetry-appender-tracing/CHANGELOG.md

@@ -2,10 +2,31 @@
 
 ## vNext
 
-- New *experimental* feature to enrich log records with attributes from active
-  tracing spans (`experimental_span_attributes`). Use
-  `OpenTelemetryTracingBridge::builder()` with `with_span_attribute_allowlist`
-  to control which span attributes are copied to log records.
+- **Add tracing span attribute enrichment.** When enabled, attributes attached
+  to active [`tracing`] spans are copied onto each emitted log record. "Span"
+  here refers to a [`tracing::span!`][tracing-span] from the [`tracing`] crate
+  (the appender's source), **not** an OpenTelemetry span.
+
+  Enrichment is **disabled by default** (no per-span overhead) and must be
+  opted into at runtime via a single builder method that accepts a
+  [`TracingSpanAttributes`] value:
+
+  ```rust
+  use opentelemetry_appender_tracing::layer::TracingSpanAttributes;
+
+  // Copy all tracing-span attributes onto log records:
+  let layer = OpenTelemetryTracingBridge::builder(&provider)
+      .with_tracing_span_attributes(TracingSpanAttributes::all())
+      .build();
+
+  // Or copy only an allowlist of attributes:
+  let layer = OpenTelemetryTracingBridge::builder(&provider)
+      .with_tracing_span_attributes(TracingSpanAttributes::allowlist(["session.id"]))
+      .build();
+  ```
+
+[`tracing`]: https://crates.io/crates/tracing
+[tracing-span]: https://docs.rs/tracing/latest/tracing/macro.span.html
 
 - Remove the `experimental_use_tracing_span_context` since
   `tracing-opentelemetry` now supports [activating][31901]  the OpenTelemetry

opentelemetry-appender-tracing/Cargo.toml

@@ -34,7 +34,6 @@ pprof = { workspace = true }
 [features]
 default = []
 experimental_metadata_attributes = ["dep:tracing-log"]
-experimental_span_attributes = []
 bench_profiling = []
 
 [[bench]]

opentelemetry-appender-tracing/benches/logs.rs

@@ -14,13 +14,14 @@
 
     Hardware: Apple M4 Pro
     Total Number of Cores:	14 (10 performance and 4 efficiency)
+    rustc 1.95.0 (59807616e 2026-04-14)
     | Test                        | Average time|
     |-----------------------------|-------------|
-    | log_no_subscriber           | 285 ps      |
-    | noop_layer_disabled         | 8 ns       |
-    | noop_layer_enabled          | 14 ns       |
-    | ot_layer_disabled           | 12 ns       |
-    | ot_layer_enabled            | 130 ns      |
+    | log_no_subscriber           | 262 ps      |
+    | noop_layer_disabled         |   4 ns      |
+    | noop_layer_enabled          |  12 ns      |
+    | ot_layer_disabled           |   7 ns      |
+    | ot_layer_enabled            | 122 ns      |
 */
 
 use criterion::{criterion_group, criterion_main, Criterion};

opentelemetry-appender-tracing/benches/span-attributes.rs

@@ -1,37 +1,39 @@
 /*
 // Run this benchmark with:
-// cargo bench --bench span-attributes --features experimental_span_attributes
+// cargo bench --bench span-attributes
 // The benchmark results:
 // Hardware: Mac M4 Pro
 // Total Number of Cores: 14 (10 performance and 4 efficiency)
-// rustc 1.93.0 (254b59607 2026-01-19)
-// cargo 1.93.0 (083ac5135 2025-12-15)
+// rustc 1.95.0 (59807616e 2026-04-14)
+// cargo 1.95.0 (f2d3ce0bd 2026-03-21)
 //
 // Attribute counts are chosen to stay within the SDK's inline-array optimisation
-// threshold (5 attributes). With `experimental_span_attributes` enabled, span
+// threshold (5 attributes). With tracing-span attribute enrichment enabled (via
+// `OpenTelemetryTracingBridgeBuilder::with_tracing_span_attributes`), tracing-span
 // attributes are copied onto the log record, so total = log attrs + span attrs.
 //
 // Log + span benchmarks (showing incremental cost of span attributes on logging):
 // | Test                                      | Total attrs | Average time | Increment vs baseline        |
 // |-------------------------------------------|-------------|--------------|------------------------------|
-// | log_1_attr_no_span                        | 1           | 81 ns        | -                            |
-// | log_1_attr_in_span_2_attr                 | 3           | 325 ns       | +244 ns                      |
-// | log_1_attr_in_nested_spans_2plus2_attr    | 5           | 570 ns       | +489 ns (+245 ns vs 1 span)  |
+// | log_1_attr_no_span                        | 1           |  85 ns       | -                            |
+// | log_1_attr_in_span_2_attr                 | 3           | 382 ns       | +297 ns                      |
+// | log_1_attr_in_nested_spans_2plus2_attr    | 5           | 590 ns       | +505 ns (+208 ns vs 1 span)  |
 //
 // Span-only benchmarks (no log emission, kept for reference):
 // | Test                  | Average time | Increment |
 // |-----------------------|--------------|-----------|
-// | span_4_attributes     | 175 ns       | -         |
-// | span_8_attributes     | 272 ns       | +97 ns    |
-// | nested_spans_1_levels | 183 ns       | -         |
-// | nested_spans_2_levels | 385 ns       | +202 ns   |
-// | nested_spans_3_levels | 583 ns       | +198 ns   |
+// | span_4_attributes     | 194 ns       | -         |
+// | span_8_attributes     | 314 ns       | +120 ns   |
+// | nested_spans_1_levels | 194 ns       | -         |
+// | nested_spans_2_levels | 413 ns       | +219 ns   |
+// | nested_spans_3_levels | 630 ns       | +217 ns   |
 
 */
 
 use criterion::{criterion_group, criterion_main, Criterion};
 use opentelemetry::InstrumentationScope;
 use opentelemetry_appender_tracing::layer as tracing_layer;
+use opentelemetry_appender_tracing::layer::TracingSpanAttributes;
 use opentelemetry_sdk::error::OTelSdkResult;
 use opentelemetry_sdk::logs::{LogProcessor, SdkLogRecord, SdkLoggerProvider};
 use opentelemetry_sdk::Resource;
@@ -67,7 +69,9 @@ fn benchmark_span_attributes(c: &mut Criterion, num_attributes: usize) {
         .with_log_processor(NoopProcessor)
         .build();
 
-    let ot_layer = tracing_layer::OpenTelemetryTracingBridge::new(&provider);
+    let ot_layer = tracing_layer::OpenTelemetryTracingBridge::builder(&provider)
+        .with_tracing_span_attributes(TracingSpanAttributes::all())
+        .build();
     let subscriber = Registry::default().with(ot_layer);
 
     tracing::subscriber::with_default(subscriber, || {
@@ -129,7 +133,9 @@ fn benchmark_nested_spans(c: &mut Criterion, depth: usize) {
         .with_log_processor(NoopProcessor)
         .build();
 
-    let ot_layer = tracing_layer::OpenTelemetryTracingBridge::new(&provider);
+    let ot_layer = tracing_layer::OpenTelemetryTracingBridge::builder(&provider)
+        .with_tracing_span_attributes(TracingSpanAttributes::all())
+        .build();
     let subscriber = Registry::default().with(ot_layer);
 
     tracing::subscriber::with_default(subscriber, || {
@@ -246,7 +252,9 @@ fn make_provider() -> SdkLoggerProvider {
 /// 5-attribute inline-array optimisation threshold.
 fn benchmark_log_1_attr_no_span(c: &mut Criterion) {
     let provider = make_provider();
-    let ot_layer = tracing_layer::OpenTelemetryTracingBridge::new(&provider);
+    let ot_layer = tracing_layer::OpenTelemetryTracingBridge::builder(&provider)
+        .with_tracing_span_attributes(TracingSpanAttributes::all())
+        .build();
     let subscriber = Registry::default().with(ot_layer);
 
     tracing::subscriber::with_default(subscriber, || {
@@ -264,11 +272,13 @@ fn benchmark_log_1_attr_no_span(c: &mut Criterion) {
 
 /// Emit an error log with 1 attribute while inside a span that carries 2 attributes
 /// (total = 3 attrs on the log record, within the SDK's 5-attr inline threshold).
-/// With `experimental_span_attributes` the 2 span attributes are propagated onto the
+/// With tracing-span attribute enrichment enabled, the 2 span attributes are copied onto the
 /// log record, so the delta vs `log_1_attr_no_span` shows the pure span-attribute cost.
 fn benchmark_log_1_attr_in_span_2_attr(c: &mut Criterion) {
     let provider = make_provider();
-    let ot_layer = tracing_layer::OpenTelemetryTracingBridge::new(&provider);
+    let ot_layer = tracing_layer::OpenTelemetryTracingBridge::builder(&provider)
+        .with_tracing_span_attributes(TracingSpanAttributes::all())
+        .build();
     let subscriber = Registry::default().with(ot_layer);
 
     tracing::subscriber::with_default(subscriber, || {
@@ -293,7 +303,9 @@ fn benchmark_log_1_attr_in_span_2_attr(c: &mut Criterion) {
 /// two spans) vs the same total number of span attributes on a single span.
 fn benchmark_log_1_attr_in_nested_spans_2plus2_attr(c: &mut Criterion) {
     let provider = make_provider();
-    let ot_layer = tracing_layer::OpenTelemetryTracingBridge::new(&provider);
+    let ot_layer = tracing_layer::OpenTelemetryTracingBridge::builder(&provider)
+        .with_tracing_span_attributes(TracingSpanAttributes::all())
+        .build();
     let subscriber = Registry::default().with(ot_layer);
 
     tracing::subscriber::with_default(subscriber, || {