opentelemetry: forward event source locations (#1911)

This branch adds the source code file, module path, and line number to
OpenTelemetry events as the OpenTelemetry `code.filepath`,
`code.namespace`, and `code.lineno` fields, respectively, if they are
set in the `tracing` event's metadata.

Fixes #1910

tracing-opentelemetry/src/layer.rs

@@ -1,9 +1,11 @@
 use crate::{OtelData, PreSampledTracer};
 use opentelemetry::{
     trace::{self as otel, noop, TraceContextExt},
-    Context as OtelContext, Key, KeyValue,
+    Context as OtelContext, Key, KeyValue, Value,
 };
 use std::any::TypeId;
+#[cfg(not(feature = "tracing-log"))]
+use std::borrow::Cow;
 use std::fmt;
 use std::marker;
 use std::time::{Instant, SystemTime};
@@ -27,6 +29,7 @@ const SPAN_STATUS_MESSAGE_FIELD: &str = "otel.status_message";
 /// [tracing]: https://github.com/tokio-rs/tracing
 pub struct OpenTelemetryLayer<S, T> {
     tracer: T,
+    event_location: bool,
     tracked_inactivity: bool,
     get_context: WithContext,
     _registry: marker::PhantomData<S>,
@@ -289,6 +292,7 @@ where
     pub fn new(tracer: T) -> Self {
         OpenTelemetryLayer {
             tracer,
+            event_location: true,
             tracked_inactivity: true,
             get_context: WithContext(Self::get_context),
             _registry: marker::PhantomData,
@@ -327,12 +331,24 @@ where
     {
         OpenTelemetryLayer {
             tracer,
+            event_location: self.event_location,
             tracked_inactivity: self.tracked_inactivity,
             get_context: WithContext(OpenTelemetryLayer::<S, Tracer>::get_context),
             _registry: self._registry,
         }
     }
 
+    /// Sets whether or not event span's metadata should include detailed location
+    /// information, such as the file, module and line number.
+    ///
+    /// By default, event locations are enabled.
+    pub fn with_event_location(self, event_location: bool) -> Self {
+        Self {
+            event_location,
+            ..self
+        }
+    }
+
     /// Sets whether or not spans metadata should include the _busy time_
     /// (total time for which it was entered), and _idle time_ (total time
     /// the span existed but was not entered).
@@ -555,6 +571,33 @@ where
                     builder.status_code = Some(otel::StatusCode::Error);
                 }
 
+                if self.event_location {
+                    let builder_attrs = builder.attributes.get_or_insert(Vec::new());
+
+                    #[cfg(not(feature = "tracing-log"))]
+                    let normalized_meta = None;
+                    let (file, module) = match &normalized_meta {
+                        Some(meta) => (
+                            meta.file().map(|s| Value::from(s.to_owned())),
+                            meta.module_path().map(|s| Value::from(s.to_owned())),
+                        ),
+                        None => (
+                            event.metadata().file().map(Value::from),
+                            event.metadata().module_path().map(Value::from),
+                        ),
+                    };
+
+                    if let Some(file) = file {
+                        builder_attrs.push(KeyValue::new("code.filepath", file));
+                    }
+                    if let Some(module) = module {
+                        builder_attrs.push(KeyValue::new("code.namespace", module));
+                    }
+                    if let Some(line) = meta.line() {
+                        builder_attrs.push(KeyValue::new("code.lineno", line as i64));
+                    }
+                }
+
                 if let Some(ref mut events) = builder.events {
                     events.push(otel_event);
                 } else {