Attach client-level span fields

This isn't quite what we want because the fields are still buried in a "self" field, despite numerous attempts to flatten them. Opened tokio-rs/tracing#3124 to track a feature request (or similar).

We have options but most are rather error prone. Perhaps the best way is our own attribute macro that would expand to `#[tracing::instrument]` with appropriate defaults to attach client-level fields like `az.namespace`.

We could also write a helper function or two to just wrap the whole thing, which would also give us better control over the error or return value as well, like attaching `error.type` per our guidelines.

.cargo/config.toml

@@ -0,0 +1,2 @@
+[build]
+rustflags = ["--cfg", "tracing_unstable"]

Cargo.toml

@@ -7,8 +7,9 @@ publish = false
 [dependencies]
 serde = { version = "1.0.213", features = ["derive"] }
 tokio = { version = "1.41.0", features = ["sync", "time"] }
-tracing = "0.1.40"
+tracing = { version = "0.1.40", features = ["valuable"] }
 url = "2.5.2"
+valuable = { version = "0.1.0", features = ["derive"] }
 
 [dev-dependencies]
 clap = { version = "4.5.20", features = ["derive"] }

src/clients.rs

@@ -9,13 +9,16 @@ use std::{collections::HashMap, fmt, sync::Arc, time::Duration};
 use tokio::{sync::Mutex, time::sleep};
 use tracing::{info_span, Instrument};
 use url::Url;
+use valuable::{Fields, NamedField, NamedValues, Structable, Valuable};
 
 pub struct ExampleClient {
     endpoint: Url,
     models: Arc<Mutex<HashMap<String, Model>>>,
 }
 
 impl ExampleClient {
+    const NAMESPACE: &str = "Microsoft.Example";
+
     pub fn new(endpoint: impl AsRef<str>) -> Result<Self> {
         Ok(Self {
             endpoint: Url::parse(endpoint.as_ref())?,
@@ -30,10 +33,11 @@ impl ExampleClient {
     pub async fn get_model(&self, name: &str) -> Result<Model> {
         let mut span = tracing::Span::current();
         if span
-            .field("client")
-            .is_none_or(|name| name.name() == "ExampleClient")
+            .field(AZ_CLIENT_FIELD.name())
+            .is_none_or(|name| name.name() == stringify!(ExampleClient))
         {
-            span = info_span!(target: "ExampleClient::get_model", "get_model", client = "ExampleClient");
+            span =
+                info_span!(target: "ExampleClient::get_model", "get_model", self = self.as_value());
         }
         async move {
             sleep(Duration::from_millis(100)).await;
@@ -54,10 +58,10 @@ impl ExampleClient {
     pub async fn create_or_update_model(&self, model: Model) -> Result<Model> {
         let mut span = tracing::Span::current();
         if span
-            .field("client")
-            .is_none_or(|name| name.name() == "ExampleClient")
+            .field(AZ_CLIENT_FIELD.name())
+            .is_none_or(|name| name.name() == stringify!(ExampleClient))
         {
-            span = info_span!(target: "ExampleClient::create_or_update_model", "create_or_update_model", client = "ExampleClient");
+            span = info_span!(target: "ExampleClient::create_or_update_model", "create_or_update_model", self = self.as_value());
         }
         async move {
             sleep(Duration::from_millis(300)).await;
@@ -75,6 +79,10 @@ impl ExampleClient {
             tracing::error!(name: "create_or_update_model", target: "ExampleClient::create_or_update_model", error = %err)
         )
     }
+
+    pub(crate) fn as_value(&self) -> valuable::Value<'_> {
+        tracing::field::valuable(self)
+    }
 }
 
 impl fmt::Debug for ExampleClient {
@@ -84,3 +92,29 @@ impl fmt::Debug for ExampleClient {
             .finish()
     }
 }
+
+impl Valuable for ExampleClient {
+    fn as_value(&self) -> valuable::Value<'_> {
+        valuable::Value::Structable(self)
+    }
+
+    fn visit(&self, visit: &mut dyn valuable::Visit) {
+        visit.visit_named_fields(&NamedValues::new(
+            FIELDS,
+            &[
+                stringify!(ExampleClient).as_value(),
+                Self::NAMESPACE.as_value(),
+            ],
+        ));
+    }
+}
+
+pub(crate) const AZ_CLIENT_FIELD: NamedField = NamedField::new("az.client");
+pub(crate) const AZ_NAMESPACE_FIELD: NamedField = NamedField::new("az.namespace");
+static FIELDS: &[NamedField<'static>] = &[AZ_CLIENT_FIELD, AZ_NAMESPACE_FIELD];
+
+impl Structable for ExampleClient {
+    fn definition(&self) -> valuable::StructDef<'_> {
+        valuable::StructDef::new_static(stringify!(ExampleClient), Fields::Named(FIELDS))
+    }
+}

src/lib.rs

@@ -18,10 +18,10 @@ pub trait ExampleClientExt: private::Sealed {
 
 impl ExampleClientExt for ExampleClient {
     #[tracing::instrument(
-        name = "rotate",
+        "rotate",
         target = "ExampleClient::rotate",
         skip_all,
-        fields(client = "ExampleClient")
+        fields(self = self.as_value())
     )]
     async fn rotate<S: Into<Secret>>(&self, name: &str, secret: S) -> Result<Model> {
         let mut m = self.get_model(name).await?;