feat(relay): add OpenTelemetry tracing, keep Prometheus metrics

Cargo.toml

@@ -65,8 +65,12 @@ serde_yaml  = "0.9"
 evalexpr    = "11"
 cron        = "0.16"
 # Observability
-tracing             = "0.1"
-tracing-subscriber  = { version = "0.3", features = ["env-filter", "json"] }
+tracing                     = "0.1"
+tracing-subscriber          = { version = "0.3", features = ["env-filter", "json"] }
+tracing-opentelemetry       = { version = "0.33" }
+opentelemetry               = { version = "0.32", features = ["trace"] }
+opentelemetry_sdk           = { version = "0.32", features = ["trace", "rt-tokio"] }
+opentelemetry-otlp          = { version = "0.32", default-features = false, features = ["trace", "grpc-tonic", "tls-ring"] }
 metrics             = "0.24"
 metrics-exporter-prometheus = "0.18"
 

crates/buzz-db/src/api_token.rs

@@ -350,7 +350,7 @@ mod tests {
         let pool = PgPool::connect(TEST_DB_URL)
             .await
             .expect("connect to test DB");
-        Db { pool }
+        Db::from_pool(pool)
     }
 
     async fn make_community(pool: &PgPool) -> Uuid {

crates/buzz-db/src/lib.rs

@@ -129,6 +129,19 @@ pub async fn insert_mentions(
 #[derive(Clone, Debug)]
 pub struct Db {
     pub(crate) pool: PgPool,
+    /// Maximum connections configured for this pool (from [`DbConfig::max_connections`]).
+    pub(crate) max_connections: u32,
+}
+
+/// Snapshot of Postgres connection pool utilisation.
+#[derive(Debug, Clone, Copy)]
+pub struct DbPoolStats {
+    /// Total connections currently in the pool (idle + active).
+    pub size: u32,
+    /// Connections available for immediate reuse.
+    pub idle: u32,
+    /// Pool ceiling — the `max_connections` value set at construction.
+    pub max: u32,
 }
 
 /// Configuration for the Postgres connection pool.
@@ -201,12 +214,18 @@ impl Db {
             .idle_timeout(Duration::from_secs(config.idle_timeout_secs))
             .connect(&config.database_url)
             .await?;
-        Ok(Self { pool })
+        Ok(Self {
+            pool,
+            max_connections: config.max_connections,
+        })
     }
 
     /// Creates a `Db` from an existing `PgPool` (useful in tests).
     pub fn from_pool(pool: PgPool) -> Self {
-        Self { pool }
+        Self {
+            max_connections: pool.options().get_max_connections(),
+            pool,
+        }
     }
 
     /// Run pending database migrations.
@@ -219,6 +238,19 @@ impl Db {
         sqlx::query("SELECT 1").execute(&self.pool).await.is_ok()
     }
 
+    /// Returns pool utilisation stats for metrics emission.
+    ///
+    /// `size`  — total connections (idle + active)
+    /// `idle`  — connections available for immediate reuse
+    /// `max`   — pool ceiling set at construction
+    pub fn pool_stats(&self) -> DbPoolStats {
+        DbPoolStats {
+            size: self.pool.size(),
+            idle: self.pool.num_idle() as u32,
+            max: self.max_connections,
+        }
+    }
+
     /// Begin a database transaction for atomic multi-statement operations.
     ///
     /// Returns a `'static` transaction because `PgPool` is `Arc`-backed internally.
@@ -2517,7 +2549,7 @@ mod tests {
         let pool = PgPool::connect(TEST_DB_URL)
             .await
             .expect("connect to test DB");
-        Db { pool }
+        Db::from_pool(pool)
     }
 
     async fn make_community(pool: &PgPool) -> Uuid {

crates/buzz-relay/Cargo.toml

@@ -36,6 +36,10 @@ serde = { workspace = true }
 serde_json = { workspace = true }
 tracing = { workspace = true }
 tracing-subscriber = { workspace = true }
+tracing-opentelemetry = { workspace = true }
+opentelemetry = { workspace = true }
+opentelemetry_sdk = { workspace = true }
+opentelemetry-otlp = { workspace = true }
 thiserror = { workspace = true }
 anyhow = { workspace = true }
 uuid = { workspace = true }