feat(tracing): add datastore tracing plumbing (#2760)

Configure a dedicated datastore tracing target on the OTLP layer while
preserving explicit logging filters and avoiding span overhead when OTLP
is disabled.

This is in preparation for adding trace spans for datastores used in
Buzz

## Update — 2026-07-27

- Export HTTP requests as `INFO` server spans under `buzz_relay`,
preserving request parentage for datastore spans.
- Configure OTEL span filtering independently with `BUZZ_OTEL_FILTER`,
so `RUST_LOG` changes cannot break trace topology.
- Verify exported HTTP and datastore spans share a trace ID and have the
expected parent/child relationship.

Co-authored-by: Amp <amp@ampcode.com>

crates/buzz-relay/Cargo.toml

@@ -92,3 +92,4 @@ reqwest = { workspace = true }
 tokio-tungstenite = { workspace = true }
 futures = "0.3"
 flate2 = "1.1.9"
+opentelemetry_sdk = { workspace = true, features = ["testing"] }

crates/buzz-relay/src/main.rs

@@ -4,6 +4,10 @@ use std::sync::Arc;
 
 use tracing::{error, info, warn};
 use tracing_subscriber::{fmt, prelude::*, EnvFilter};
+
+fn log_env_filter(rust_log: Option<&str>) -> EnvFilter {
+    EnvFilter::new(rust_log.unwrap_or("buzz_relay=info"))
+}
 use uuid::Uuid;
 
 use buzz_audit::AuditService;
@@ -107,9 +111,17 @@ async fn main() -> anyhow::Result<()> {
     };
 
     tracing_subscriber::registry()
-        .with(fmt::layer().json().flatten_event(true))
-        .with(EnvFilter::from_default_env().add_directive("buzz_relay=info".parse()?))
-        .with(otel_layer)
+        .with(
+            fmt::layer()
+                .json()
+                .flatten_event(true)
+                .with_filter(log_env_filter(std::env::var("RUST_LOG").ok().as_deref())),
+        )
+        .with(otel_layer.map(|layer| {
+            layer.with_filter(telemetry::otel_env_filter(
+                std::env::var("BUZZ_OTEL_FILTER").ok().as_deref(),
+            ))
+        }))
         .init();
 
     // Log any exporter-build failure now that the subscriber is installed.
@@ -1060,6 +1072,52 @@ async fn main() -> anyhow::Result<()> {
     Ok(())
 }
 
+#[cfg(test)]
+mod env_filter_tests {
+    use super::log_env_filter;
+    use buzz_relay::telemetry::otel_env_filter;
+    use tracing_subscriber::prelude::*;
+
+    #[test]
+    fn unset_enables_datastore_only_for_otel_filter() {
+        let logs = tracing_subscriber::registry().with(log_env_filter(None));
+        tracing::subscriber::with_default(logs, || {
+            assert!(!tracing::enabled!(target: "buzz_datastore", tracing::Level::INFO));
+            assert!(tracing::enabled!(target: "buzz_relay", tracing::Level::INFO));
+        });
+
+        let otel = tracing_subscriber::registry().with(otel_env_filter(None));
+        tracing::subscriber::with_default(otel, || {
+            assert!(tracing::enabled!(target: "buzz_datastore", tracing::Level::INFO));
+        });
+    }
+
+    #[test]
+    fn explicit_datastore_off_is_preserved_alone() {
+        assert_eq!(
+            otel_env_filter(Some("buzz_datastore=off")).to_string(),
+            "buzz_datastore=off"
+        );
+    }
+
+    #[test]
+    fn explicit_datastore_debug_is_preserved_alone() {
+        assert_eq!(
+            otel_env_filter(Some("buzz_datastore=debug")).to_string(),
+            "buzz_datastore=debug"
+        );
+    }
+
+    #[test]
+    fn log_and_otel_filters_are_configured_independently() {
+        assert_eq!(log_env_filter(Some("warn")).to_string(), "warn");
+        assert_eq!(
+            otel_env_filter(Some("buzz_relay=debug")).to_string(),
+            "buzz_relay=debug"
+        );
+    }
+}
+
 async fn run_community_revalidator(
     state: Arc<AppState>,
     period: std::time::Duration,

crates/buzz-relay/src/router.rs

@@ -4,8 +4,9 @@ use std::sync::atomic::Ordering;
 use std::sync::Arc;
 
 use axum::{
+    body::Body,
     extract::{ConnectInfo, FromRequest, State, WebSocketUpgrade},
-    http::{HeaderMap, StatusCode},
+    http::{HeaderMap, Request, StatusCode},
     middleware,
     response::{IntoResponse, Json},
     routing::{get, post, put},
@@ -16,7 +17,7 @@ use tower::ServiceExt;
 use tower_http::cors::{AllowOrigin, CorsLayer};
 use tower_http::limit::RequestBodyLimitLayer;
 use tower_http::services::ServeDir;
-use tower_http::trace::TraceLayer;
+use tower_http::trace::{HttpMakeClassifier, TraceLayer};
 
 use crate::api;
 use crate::audio;
@@ -187,10 +188,23 @@ pub fn build_router(state: Arc<AppState>) -> Router {
 
     merged
         .layer(middleware::from_fn(track_metrics))
-        .layer(TraceLayer::new_for_http())
+        .layer(http_trace_layer())
         .layer(build_cors_layer(&state.config.cors_origins))
 }
 
+fn http_trace_layer() -> TraceLayer<HttpMakeClassifier, fn(&Request<Body>) -> tracing::Span> {
+    TraceLayer::new_for_http().make_span_with(make_http_span as fn(&Request<Body>) -> tracing::Span)
+}
+
+fn make_http_span(request: &Request<Body>) -> tracing::Span {
+    tracing::info_span!(
+        target: "buzz_relay",
+        "http.request",
+        otel.kind = "server",
+        http.request.method = %request.method(),
+    )
+}
+
 fn is_admin_spa_path(path: &str) -> bool {
     path == "/"
         || path == "/reports"
@@ -435,9 +449,14 @@ fn build_cors_layer(cors_origins: &[String]) -> CorsLayer {
 mod tests {
     use axum::{routing::get, Router};
     use futures_util::SinkExt;
+    use opentelemetry::trace::TracerProvider as _;
+    use opentelemetry_sdk::trace::{InMemorySpanExporter, SdkTracerProvider};
     use tokio::net::TcpListener;
     use tokio::sync::mpsc;
     use tokio_tungstenite::{connect_async, tungstenite::Message};
+    use tower::ServiceBuilder;
+    use tracing::Instrument as _;
+    use tracing_subscriber::prelude::*;
 
     use super::*;
 
@@ -471,6 +490,60 @@ mod tests {
         assert!(!should_serve_spa("/arbitrary", true));
     }
 
+    #[tokio::test(flavor = "current_thread")]
+    async fn http_and_datastore_spans_are_exported_in_the_same_trace() {
+        let exporter = InMemorySpanExporter::default();
+        let provider = SdkTracerProvider::builder()
+            .with_simple_exporter(exporter.clone())
+            .build();
+        let subscriber = tracing_subscriber::registry().with(
+            tracing_opentelemetry::layer()
+                .with_tracer(provider.tracer("test"))
+                .with_filter(crate::telemetry::otel_env_filter(None)),
+        );
+        let _subscriber_guard = tracing::subscriber::set_default(subscriber);
+        let service = ServiceBuilder::new()
+            .layer(http_trace_layer())
+            .service(tower::service_fn(
+                |_: axum::http::Request<axum::body::Body>| async {
+                    async {}
+                        .instrument(tracing::info_span!(
+                            target: "buzz_datastore",
+                            "SELECT",
+                            otel.kind = "client",
+                            db.system.name = "postgresql",
+                        ))
+                        .await;
+                    Ok::<_, std::convert::Infallible>(axum::response::Response::new(
+                        axum::body::Body::empty(),
+                    ))
+                },
+            ));
+
+        service
+            .oneshot(
+                axum::http::Request::get("/")
+                    .body(axum::body::Body::empty())
+                    .unwrap(),
+            )
+            .await
+            .unwrap();
+
+        provider.force_flush().unwrap();
+        let spans = exporter.get_finished_spans().unwrap();
+        let http = spans
+            .iter()
+            .find(|span| span.name == "http.request")
+            .unwrap();
+        let datastore = spans.iter().find(|span| span.name == "SELECT").unwrap();
+
+        assert_eq!(
+            datastore.span_context.trace_id(),
+            http.span_context.trace_id()
+        );
+        assert_eq!(datastore.parent_span_id, http.span_context.span_id());
+    }
+
     async fn handler_receives_message_with_limit(limit: usize, size: usize) -> bool {
         let (received_tx, mut received_rx) = mpsc::unbounded_channel();
         let app = Router::new().route(

crates/buzz-relay/src/telemetry.rs

@@ -25,6 +25,16 @@
 
 use opentelemetry_otlp::ExporterBuildError;
 use opentelemetry_sdk::{resource::EnvResourceDetector, trace::SdkTracerProvider, Resource};
+use tracing_subscriber::EnvFilter;
+
+/// Build the filter for spans exported through OpenTelemetry.
+///
+/// This is intentionally independent from `RUST_LOG`: changing stdout log
+/// verbosity must not remove parent spans from exported traces. Set
+/// `BUZZ_OTEL_FILTER` to override the default targets.
+pub fn otel_env_filter(configured: Option<&str>) -> EnvFilter {
+    EnvFilter::new(configured.unwrap_or("buzz_relay=info,buzz_datastore=info"))
+}
 
 /// Build the OTEL [`Resource`] used by the trace provider.
 ///